Eye

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Anatomy of the Human Eye – Parts of the Eye Explained

Parts of the Eye Explained

From the moment your eyes open in the morning, to when you turn off the lights to sleep at night, your eyes are taking in information so you are aware of your surroundings at all times. How often do you take your eyesight for granted? The eyes play an essential role in everyday activities, even when you don’t know what makes the eyes work or how your eyes and brain are processing images.

Eyes: A Built-In Camera

Your eyes work in a similar way as a camera. Pictures are captured and sent to the brain, where the information is processed and categorized. As the light passes through your eye, it is picked up in the back area of the eye. Then, this picture is changed to a message that can be sent to a part of the brain which handles visual processing.

For this process to work, multiple parts of the eye need to be working together. In the same way that a camera will malfunction if the lens is blocked or a part is broken, your eyes need to have functional parts for your vision to be accurate and clear.

 Parts of the Eye (And What They Do)

If you want to understand how various conditions and diseases affect the eye, then you need to know how the eyes work. Here is an overview of the many important parts within your eye and how they impact your vision:

  • Orbit: The eye is located in a bony socket within the skull, known as an orbit. Six extraocular muscles are located in the orbit, which attaches to the eye to enable movement. The muscles allow you to look side-to-side, up and down, and rotate the eye. 
  • Sclera: The white, visible area of the eyeball is called the sclera. It is durable and acts as a protective layer, covering most of the eyeball surface. The extraocular muscles are attached to the sclera.  
  • Conjunctiva: A thin layer of tissue covers most of the front surface of your eye. The conjunctiva also acts as a lining inside of your eyelid. When the eyes look red, it means that the blood vessels within the conjunctiva are bigger than normal.
  • Cornea: The front of the eye holds the cornea, which is like a glass lens that moves light into the eye. This clear part of the eye focuses the light so the image can reach the back of the eye. When you look closely at an eye, the cornea is the clear, bulging surface that forms the shape for the front of the eye.
  • Lens: This part of the eye works along with the cornea to focus the light on the retina (located in the back of the eye). The lens can change shape by getting thicker or thinner to optimize the clarity of the picture as it hits the retina.
  • Ciliary Muscles: Around the lens, a circle of small muscles works to change the shape of the lens. These muscles relax or squeeze depending on the distance of the object. For example, if you are looking at something up close, then the ciliary muscles make the lens fat by squeezing the lens. The muscles relax to thin the lens when you are focusing on distant objects.
  • Retina: The retina is located in the back section of the eye and contains rods and cones, helping you distinguish between colors and black and white. This area of the eye is actually a group of light-sensing cells, collectively referred to as the retina. When the image you are seeing is in the retina, it is turned into an electrical message that is sent to the brain.
  • Macula: Within the retina is a small, specialized area known as the macula. This part of the eye helps with central vision so you can see details in the various images.
  • Optic Nerve: When the electrical messages are received in the retina, they are sent along the optic nerve to reach the brain.
  • Vitreous Humor: In the space between the retina and lens, this area is filled with a transparent, jelly-like substance. It helps to maintain the shape of the eye and protect the structure of the eye.
  • Iris: Every person has a unique eye color that can be seen in the middle of the eye. This pigmented area is known as the iris, and the color is inherited genetically. Not only is the iris beautiful, but it is also functional to control the amount of light that can enter into the eye.
  • Pupil: In the center of the iris is the pupil, which is where the light enters in. When you are in a dark environment, the pupils get bigger to optimize your vision. In brighter environments, the pupils get smaller to manage the amount of light that comes inside. The diameter of the pupil usually ranges between 3 and 7 mm, depending on the light conditions.
  • Eyelids: The skin covering that moves over the eye is important as a protective mechanism. Eyelids block the light when you are sleeping and have an instinctual mechanism to blink, so the eyes stay moisturized.
  • Tear Glands: In the upper eyelid, small glands are there to provide the tears that keep the surface of the eyes moist and clean. This moisture is important for protecting your eye from damage.
  • Eyelashes and Eyebrows: The lashes and brows around your eye work to block dust and sweat from getting in your eyes.

The different parts of the eye are all unique, but each is an important piece of a whole system to help you see.

 

How Eye Changes Affect Your Vision

As changes occur in one or more parts of the eye, it can have an impact on your vision. For example, when the curve of the cornea changes, it affects the way the light is reflected on the retina, which in turn makes it difficult to focus on the things that you are seeing. Other eye conditions can impact vision as well. Corrective lenses can be worn, such as glasses or contact lenses, which adjust the way the light enters the eye and hits the retina. Or, some people choose laser eye surgery to reshape the cornea.

If you are ready to get rid of your glasses and contacts, then contact us to learn more about the benefits of LASIK eye surgery. Our team at LasikPlus will discuss your options and help you find the right provider to assist with your eye care. Schedule an appointment online or call us at 1.866.755.2026.

Your Eyes (for Kids) – Nemours Kidshealth

Which part of your body lets you read the back of a cereal box, check out a rainbow, and see a softball heading your way? Which part lets you cry when you’re sad and makes tears to protect itself? Which part has muscles that adjust to let you focus on things that are close up or far away? If you guessed the eye, you’re right!

Your eyes are at work from the moment you wake up to the moment you close them to go to sleep. They take in tons of information about the world around you — shapes, colors, movements, and more. Then they send the information to your brain for processing so the brain knows what’s going on outside of your body.

You can see that the eye’s pretty amazing. So, come on — let’s take a tour of its many parts.

The Parts of the Eye

You can check out different parts of the eye by looking at your own eye in the mirror or by looking at (but not touching) a friend’s eye. Some of the eye’s parts are easy to see, so most friends will say OK. Most friends won’t say OK if you ask to see their liver!

Big as a Ping Pong Ball

The eye is about as big as a ping-pong ball and sits in a little hollow area (the eye socket) in the skull. The eyelid protects the front part of the eye. The lid helps keep the eye clean and moist by opening and shutting several times a minute. This is called blinking, and it’s both a voluntary and involuntary action, meaning you can blink whenever you want to, but it also happens without you even thinking about it.

The eyelid also has great reflexes, which are automatic body responses, that protect the eye. When you step into bright light, for example, the eyelids squeeze together tightly to protect your eyes until they can adjust to the light. And if you flutter your fingers close (but not too close!) to your friend’s eyes, you’ll be sure to see your friend’s eyes blink. Your friend’s eyelids shut automatically to protect the eye from possible danger. And speaking of fluttering, don’t forget eyelashes. They work with the eyelids to keep dirt and other unwanted stuff out of your eyes.

The white part of the eyeball is called the sclera (say: SKLAIR-uh). The sclera is made of a tough material and has the important job of covering most of the eyeball. Think of the sclera as your eyeball’s outer coat. Look very closely at the white of the eye, and you’ll see lines that look like tiny pink threads. These are blood vessels, the tiny tubes that deliver blood, to the sclera.

The cornea (say: KOR-nee-uh), a transparent dome, sits in front of the colored part of the eye. The cornea helps the eye focus as light makes its way through. It is a very important part of the eye, but you can hardly see it because it’s made of clear tissue. Like clear glass, the cornea gives your eye a clear window to view the world through.

Iris Is The Colorful Part

Behind the cornea are the iris, the pupil, and the anterior chamber. The iris (say: EYE-riss) is the colorful part of the eye. When we say a person has blue eyes, we really mean the person has blue irises! The iris has muscles attached to it that change its shape. This allows the iris to control how much light goes through the pupil (say: PYOO-pul).

The pupil is the black circle in the center of the iris, which is really an opening in the iris, and it lets light enter the eye. To see how this works, use a small flashlight to see how your eyes or a friend’s eyes respond to changes in brightness. The pupils will get smaller when the light shines near them and they’ll open wider when the light is gone.

The anterior (say: AN-teer-ee-ur) chamber is the space between the cornea and the iris. This space is filled with a special transparent fluid that nourishes the eye and keeps it healthy.

Light, Lens, Action

These next parts are really cool, but you can’t see them with just your own eyes! Doctors use special microscopes to look at these inner parts of the eye, such as the lens. After light enters the pupil, it hits the lens. The lens sits behind the iris and is clear and colorless. The lens’ job is to focus light rays on the back of the eyeball — a part called the retina (say: RET-i-nuh).

The lens works much like the lens of a movie projector at the movies. Next time you sit in the dark theater, look behind you at the stream of light coming from the projection booth. This light goes through a powerful lens, which is focusing the images onto the screen, so you can see the movie clearly. In the eye’s case, however, the film screen is your retina.

Your retina is in the very back of the eye. It holds millions of cells that are sensitive to light. The retina takes the light the eye receives and changes it into nerve signals so the brain can understand what the eye is seeing.

A Muscle Makes It Work

The lens is suspended in the eye by a bunch of fibers. These fibers are attached to a muscle called the ciliary (say: SIL-ee-air-ee) body. It has the amazing job of changing the shape of the lens. That’s right — the lens actually changes shape right inside your eye! Try looking away from your computer and focusing on something way across the room. Even though you didn’t feel a thing, the shape of your lenses changed. When you look at things up close, the lens becomes thicker to focus the correct image onto the retina. When you look at things far away, the lens becomes thinner.

The biggest part of the eye sits behind the lens and is called the vitreous (say: VIH-tree-us) body. The vitreous body forms two thirds of the eye’s volume and gives the eye its shape. It’s filled with a clear, jelly-like material called the vitreous humor. Ever touch toy eyeballs in a store? Sometimes they’re kind of squishy — that’s because they’re made to feel like they’re filled with vitreous humor. In a real eye, after light passes through the lens, it shines straight through the vitreous humor to the back of the eye.

Rods and Cones Process Light

The retina uses special cells called rods and cones to process light. Just how many rods and cones does your retina have? How about 120 million rods and 7 million cones — in each eye!

Rods see in black, white, and shades of gray and tell us the form or shape that something has. Rods can’t tell the difference between colors, but they are super-sensitive, allowing us to see when it’s very dark.

Cones sense color and they need more light than rods to work well. Cones are most helpful in normal or bright light. The retina has three types of cones. Each cone type is sensitive to one of three different colors — red, green, or blue — to help you see different ranges of color. Together, these cones can sense combinations of light waves that enable our eyes to see millions of colors.

Helping You See It All

Rods and cones process the light to give you the total picture. You’re able to see that your friend has brown skin and is wearing a blue hat while he tosses an orange basketball.

Sometimes someone’s eyeball shape makes it difficult for the cornea, lens, and retina to work perfectly as a team. When this happens, some of what the person sees will be out of focus.

To correct this fuzzy vision, many people, including many kids, wear glasses. Glasses help the eyes focus images correctly on the retina and allow someone to see clearly. As adults get older, their eyes lose the ability to focus well and they often need glasses to see things up close or far away. Most older people you know — like your grandparents — probably wear glasses.

To the Brain!

Think of the optic nerve as the great messenger in the back of your eye. The rods and cones of the retina change the colors and shapes you see into millions of nerve messages. Then, the optic nerve carries those messages from the eye to the brain!

The optic nerve serves as a high-speed telephone line connecting the eye to the brain. When you see an image, your eye “telephones” your brain with a report on what you are seeing so the brain can translate that report into “cat,” “apple,” or “bicycle,” or whatever the case may be.

Have No Fear, You Have Tears

For crying out loud, the eye has its own special bathing system — tears! Above the outer corner of each eye are the lacrimal (say: LAK-ruh-mul) glands, which make tears. Every time you blink your eye, a tiny bit of tear fluid comes out of your upper eyelid. It helps wash away germs, dust, or other particles that don’t belong in your eye.

Tears also keep your eye from drying out. Then the fluid drains out of your eye by going into the lacrimal duct (this is also called the tear duct). You can see the opening of your tear duct if you very gently pull down the inside corner of your eye. When you see a tiny little hole, you’ve found the tear duct.

Your eyes sometimes make more tear fluid than normal to protect themselves. This may have happened to you if you’ve been poked in the eye, if you’ve been in a dusty or smoking area, or if you’ve been near someone who’s cutting onions.

And how about the last time you felt sad, scared, or upset? Your eyes got a message from your brain to make you cry, and the lacrimal glands made many, many tears.

Your eyes do some great things for you, so take these steps to protect them:

  • Wear protective goggles in classes where debris or chemicals could go flying, such as wood shop, metal shop, science lab, or art.
  • Wear eye protection when playing racquetball, hockey, skiing, or other sports that could injure your eyes.
  • Wear sunglasses. Too much light can damage your eyes and cause vision problems later in life. For instance, a lens could get cloudy, causing a cataract. A cataract prevents light from reaching the retina and makes it difficult to see.

The eyes you have will be yours forever — treat them right and they’ll never be out of sight!

Eye Anatomy and How the Eye Works

While the eye is an extremely complex organ, it functions optically much like a camera system. For the sake of discussing common eye diseases, the following pictures can be referred to.

Schematic drawing of the eye

BASIC ANATOMY OF THE EYE

  • Cornea: Front part or “window” of the eye.
  • Pupil: Regulate amount of light entering the eye.
  • Iris: “Colored” part of the eye.
  • Lens: Part of the eye that focuses images onto the retina.
  • Retina: Innermost layer of the eye composed of light sensitive cells which pick up the images seen by the eye.
  • Macula: That part of the retina responsible for central or “eagle eye” vision.
  • Optic Nerve: Collection of nerve endings attached to the retina connecting the eyeball to the “seeing” centers of the brain.

HOW DOES THE EYE WORK? 

The eye is our most important organ for finding out about the world around us! Sight is considered our most precious sense, and people fear blindness more than any other disability. The human eye measures only about 1 inch in diameter, and yet it can see stars billions of light years away or tiny grains of sand. It can adjust its focus between a distant point and a near one.

The eye is extremely delicate, and like other organ systems in the body, it operates under very narrow physiologic parameters where temperature, blood flow, and other factors are delicately balanced. The eyeball is only one part a complex visual system which is composed of the eye and surrounding structures, the brain, and all the connecting nerve pathways between it and the brain.

The eyeball is set in a protective cone-shaped cavity in the skull called the orbit. Fatty tissue inside the orbit surrounds the eye and cushions it against blows and other injuries. The soft tissue also enables the eye to turn easily in the orbit. Six muscles move the eyeball and help coordinate it with the other. Other important structures include the eyelids which protect and lubricate the eye; the conjunctiva or lining of the eye; the lacrimal gland which secretes tears and other protective substances; the lacrimal sac which drains away tears and other debris. The internal eye is filled a fluid type substance which nourishes and protects internal structures. The space between the cornea and lens is called the anterior chamber and is filled with a watery fluid called aqueous. The cavity between the lens and retina is called the vitreous cavity because it is filled with a jelly-like substance called vitreous.

The eye functions much like a camera system. The goal of a camera is to take light rays and focus them onto a surface so that an image can be formed. It uses a lens system for proper focusing and an apparatus for regulating the amount of light entering the camera. Improper focusing will cause a blurred image, and too much or too little light will cause an overexposed or underexposed image. In much of the same way, the eye has a focusing system, an apparatus that regulates the amount of light, and a surface upon which images are formed.

Here’s how it works:

The focusing parts of the eye—the cornea and the lens—bend light rays toward one another. This is called refraction. The cornea provides most of the refracting power of the eye which does not change because the corneal curve remains constant. The refracting power of the lens, however, constantly changes as the eye shifts back and forth between nearby and distant objects much like the automatic focusing system in a camera. This process is called “accommodation,” and is achieved by the ciliary body (a large muscle inside the eye) causing a change in the shape of the lens in response to visual demand.

Together, the cornea and lens work to bend light rays together to form a focal point on the retina. The retina is an electrically sensitive membrane which picks up the light rays, and through as series of complex mechanisms, organizes and packages these impulses. They are then sent via the optic nerve to the brain which acts much like a computer, where the information is “decoded” into what we know as vision.

The center part of the retina is called the macula. This is the most sensitive part of the retina where central vision occurs. All other retinal tissue serves the peripheral part of one’s vision. Many people erroneously believe that the pupil is responsible for focusing. The pupil only regulates the amount of light entering the eye and also affects depth of focus much like a camera system. In bright light, the pupil is small and depth of focus is greater than with a widely dilated pupil seen in dim light where the depth of focus is shallow.

Definition, Parts Of The Eye & Eye Health

Overview

What is vision?

Your vision is what allows you to see the world around you. You have vision thanks to several components within t your eye and brain that work together. These parts include the:

  • Lens.
  • Retina.
  • Optic nerve.

Each part turns light and electrical signals into images that you can see.

Anatomy

What parts of your eye make up vision?

There are many different parts of your eye and brain that work together to help you see. The main components of your vision include:

  • Cornea: This is the front layer of your eye. The cornea is dome-shaped and it works by bending the light that enters your eye.
  • Pupil: The pupil is the black dot in the center of your eye that acts as a gateway for light. It expands in dim light and shrinks in bright light. It’s controlled by the iris.
  • Iris: This part is typically referred to as your eye color. The iris is a muscle that controls the size of your pupil and the amount of light that enters your eye.
  • Lens: The lens is behind the iris and pupil. It works with your cornea to focus the light that enters your eye, much like a camera. The lens brings the image in front of you into a sharp focus, which allows you to see the details clearly.
  • Retina: Located at the back of the eye, the retina is a layer of tissue that transforms the light coming into your eye into electrical signals. These signals are sent to the brain where they are recognized as images.
  • Optic nerve: This part of your vision works as the connecting element between the retina and the brain. Your optic nerve transmits the electrical signals formed in the retina to the brain. Once there, the brain creates images.
  • Tears: Though they are most commonly thought of in relation to crying, tears are meant to keep your eyes wet and help you focus clearly. They also help protect your eyes from irritation and infection.

Conditions and Disorders

What conditions could affect my vision?

There are many different conditions that can affect your vision. These conditions often interfere with the ability of light to pass from the eye to the brain. Healthcare providers can often prevent or correct many of these conditions.Conditions that affect your vision can include:

  • Aging: As you get older, your risk increases for vision-impairing conditions. Common disorders include cataracts (clouding of the eye lens) and age-related macular degeneration (AMD), a condition that causes loss or distortion of vision.
  • Damage: Injuries may cause a detached retina or a clouding of the cornea or lens. This damage can block light from passing through your eye and cause vision loss.
  • Development disorders: Sight problems such as amblyopia (lazy eye) occur when one or both eyes develop abnormally during childhood.
  • Disease: Diseases like glaucoma (increased fluid pressure in the eye) can damage the optic nerve. As a result, they impair the brain’s ability to turn electrical signals into images.
  • Infection: Infections in any part of the eye can affect your ability to see.
  • Refractive errors: Vision problems can occur when your eye doesn’t bend light properly. This issue may impair your eye’s ability to focus and cause unclear eyesight. Corrective lenses, such as glasses or contact lenses, can often improve your eyes ability to see clearly.

Care

How can I keep my vision healthy?

There are several things to do on a daily basis to promote healthy vision. Some of these tips include:

  • Getting regular eye exams: Your eye doctor can identify and treat eye problems early. It’s important to schedule yearly eye care appointments, so any developing issues can be cared for as early as possible.
  • Wearing sunglasses: More than just a fashion statement, sunglasses protect your eyes from the sun’s damaging rays and can slow down the aging process of your eyes.
  • Wearing eye protection: If you have a job or activity where you could get an eye injury, always wear eye protection. This could include various sports, construction work or factory work.
  • Eating a healthy diet: Pick foods that are good for your eyes, such as fruits, vegetables, and salmon. Leafy greens (spinach, kale and collard greens) are especially healthy for your eyes.
  • Exercising regularly: Making time to regularly exercise can help to prevent a variety of health issues throughout your life. These can include diabetes and high blood pressure, which can cause vision problems.
  • Avoid smoking: Not smoking can reduce your risk of developing diseases like cataracts and macular degeneration.

Frequently Asked Questions

Are there different types of doctors for eye care?

There are two types of eye doctors, and they are different from your primary care physician. An optometrist is a doctor of optometry who treats vision and eye health problems. An ophthalmologist is a medical doctor who treats these issues and also performs eye surgery.

How will my doctor test my vision?

Your doctor will do an eye exam in an office setting during a regular appointment. There may be several tests during this appointment. You may be asked to cover one eye at a time and read a chart. Your doctor may also give you eye drops to dilate your eyes. This increases the size of your pupils. This test allows your doctor to see any signs of damage or disease in your retina or optic nerve.

When should I call my doctor about my vision?

Seek emergency medical treatment at an emergency room if you experience sudden vision loss. Sudden loss of vision can be a sign of a serious medical problem. Call your eye doctor if you experience sudden blurry vision or flashes of light or if blurry vision interferes with your daily activities. If you have a family history of vision problems, you should have eye exams every year to monitor your eye health and vision.

Understanding the Different Parts of Your Eye

You know that your eyes allow you to see and that several different factors affect good or poor eye health. But what else do you know about your eyes? Do you know what parts comprise this amazing organ or how they aid in vision?

Below, we discuss the different parts of the eye, as well as what issues can affect some of these components and therefore impact your ocular health.

Anterior Chamber

The anterior chamber rests behind your cornea but in front of your lens and iris. It holds the aqueous humor and allows it to drain properly from your eyes into your bloodstream.

Aqueous Humor

This thick fluid rests in the anterior chamber and provides nutrients to these two parts of your eyes. The liquid must drain regularly, and your body replaces it.

However, if you develop glaucoma, this fluid will build up, creating an uncomfortable pressure in your eyes.

Choroid

This small, vascular layer sits between your eye’s sclera and retina. It provides the outer layers of the retina with nourishment (through blood vessels) and oxygen. While you won’t develop health issues in the choroid, this component refracts light, causing the red-eye effect in photos.

Ciliary Body

The ciliary body sits between your choroid and iris, and it produces the aqueous humor and holds the lens in place.

Conjunctiva

This clear membrane covers the white portion of your eye, or the sclera. The conjunctiva also covers the inside of your eyelids. It produces mucus and tears to lubricate your eyes and keeps microbes out of your eyes.

If this thin membrane becomes inflamed or swollen, you likely have conjunctivitis, commonly known as pink eye. Other eye conditions that affect the conjunctiva include pinguecula, pterygium, and subconjunctival hemorrhages.

Cornea

Your cornea is a clear covering that rests over your pupil, iris, and anterior chamber. It provides most of your eye’s optical power. The cornea refracts light and helps your eyes focus on objects in your line of sight.

Eye issues that relate to your cornea include astigmatism, corneal abrasions, keratitis, keratoconus, and pterygium.

Fovea

The fovea is a small depression in your retina that contains cones to aid in proper eyesight. If you have problems with the fovea or the cones in it, you could develop blurry vision.

Iris

The iris is the colored portion of your eye. It is made up of a fibrovascular tissue called the stroma. The stroma connects to a muscle that allows your pupils to contract and dilate.

Developing a disease in the iris is rare, but you could still contract some conditions that affect your eye’s intraocular pressure, and, indirectly, your vision.

Lens

This part of your eye is a transparent structure inside your eye. It’s about the shape of a lentil, and it can curve both inward and outward. Like the cornea, your lens refracts light. The lens is held in place by a fibrous membrane called the zonule of Zinn or the suspensory ligaments of the lens.

If the lens has an irregular curve to it, then you’re likely to develop astigmatism. Another vision condition involving the lens is cataracts, where the lens becomes opaque or cloudy and impairs vision.

Macula

This part of your eye is close to the center of your retina. The macula allows you to see objects in great detail. As you age, you could develop macular degeneration, a disease that can cause vision problems or lead to vision loss.

Optic Nerve

This nerve carries electrical impulses from the rods and cones in the retina to the visual cortex in your brain. Without the optic nerve, your other eye components cannot send images to your brain and produce your sense of sight.

Pupil

Your pupil is the black circle in the center of your iris. It regulates how much light enters your eye. Interestingly, the pupil appears black because this tissue absorbs most of the light that passes through it.

Retina

Your retina is a sensory membrane that covers the entire back surface of your eye. When your lens picks up images, these images are sent to the retina. The retina then changes these images into signals that the optic nerve then pulses to your brain.

Some ocular issues that affect the retina include diabetic retinopathy, retinal detachment, retinitis pigmentosa, and retinoblastoma.

Sclera

The sclera is more commonly known as the whites of your eyes. This fibrous layer contains collagen and protects the inner components of your eye from damage.

Trabecular Meshwork

This component is located at the base of your cornea. It drains the aqueous humor from your eye through the anterior chamber. Using tubes known as Schlemm’s canal, the trabecular meshwork lets fluid drain into your blood system.

If the trabecular meshwork can’t properly drain the aqueous humor, you could be at risk for glaucoma.

Vitreous Humor

This transparent, gelatinous material sits between your lens and retina. It also lines the back of your eye. The vitreous humor contains cells called phagocytes that remove debris from your eye so you don’t develop eye infections.

 

Now that you understand more about what parts comprise your eye and how the organ functions as a whole, you can take more measures to protect your eyesight. If you have problems seeing or experience any other issues with your eyes, get in touch with an eye doctor from All About Eyes.

Physics of the Eye | Physics II

Learning Objectives

By the end of this section, you will be able to:

  • Explain the image formation by the eye.
  • Explain why peripheral images lack detail and color.
  • Define refractive indices.
  • Analyze the accommodation of the eye for distant and near vision.

The eye is perhaps the most interesting of all optical instruments. The eye is remarkable in how it forms images and in the richness of detail and color it can detect. However, our eyes commonly need some correction, to reach what is called “normal” vision, but should be called ideal rather than normal. Image formation by our eyes and common vision correction are easy to analyze with the optics discussed in Geometric Optics.

Figure 1. The cornea and lens of an eye act together to form a real image on the light-sensing retina, which has its densest concentration of receptors in the fovea and a blind spot over the optic nerve. The power of the lens of an eye is adjustable to provide an image on the retina for varying object distances. Layers of tissues with varying indices of refraction in the lens are shown here. However, they have been omitted from other pictures for clarity.

Figure 1 shows the basic anatomy of the eye. The cornea and lens form a system that, to a good approximation, acts as a single thin lens. For clear vision, a real image must be projected onto the light-sensitive retina, which lies at a fixed distance from the lens. The lens of the eye adjusts its power to produce an image on the retina for objects at different distances. The center of the image falls on the fovea, which has the greatest density of light receptors and the greatest acuity (sharpness) in the visual field. The variable opening (or pupil) of the eye along with chemical adaptation allows the eye to detect light intensities from the lowest observable to 1010 times greater (without damage). This is an incredible range of detection. Our eyes perform a vast number of functions, such as sense direction, movement, sophisticated colors, and distance. Processing of visual nerve impulses begins with interconnections in the retina and continues in the brain. The optic nerve conveys signals received by the eye to the brain.

Refractive indices are crucial to image formation using lenses. Table 1 shows refractive indices relevant to the eye. The biggest change in the refractive index, and bending of rays, occurs at the cornea rather than the lens. The ray diagram in Figure 2 shows image formation by the cornea and lens of the eye. The rays bend according to the refractive indices provided in Table 1. The cornea provides about two-thirds of the power of the eye, owing to the fact that speed of light changes considerably while traveling from air into cornea. The lens provides the remaining power needed to produce an image on the retina. The cornea and lens can be treated as a single thin lens, even though the light rays pass through several layers of material (such as cornea, aqueous humor, several layers in the lens, and vitreous humor), changing direction at each interface. The image formed is much like the one produced by a single convex lens. This is a case 1 image. Images formed in the eye are inverted but the brain inverts them once more to make them seem upright.

Table 1. Refractive Indices Relevant to the Eye
Material Index of Refraction
Water 1.33
Air 1.0
Cornea 1.38
Aqueous humor 1.34
Lens 1.41 average (varies throughout the lens, greatest in center)
Vitreous humor 1.34

Figure 2. An image is formed on the retina with light rays converging most at the cornea and upon entering and exiting the lens. Rays from the top and bottom of the object are traced and produce an inverted real image on the retina. The distance to the object is drawn smaller than scale.

As noted, the image must fall precisely on the retina to produce clear vision—that is, the image distance di must equal the lens-to-retina distance. Because the lens-to-retina distance does not change, the image distance di must be the same for objects at all distances. The eye manages this by varying the power (and focal length) of the lens to accommodate for objects at various distances. The process of adjusting the eye’s focal length is called accommodation. A person with normal (ideal) vision can see objects clearly at distances ranging from 25 cm to essentially infinity. However, although the near point (the shortest distance at which a sharp focus can be obtained) increases with age (becoming meters for some older people), we will consider it to be 25 cm in our treatment here.

Figure 3 shows the accommodation of the eye for distant and near vision. Since light rays from a nearby object can diverge and still enter the eye, the lens must be more converging (more powerful) for close vision than for distant vision. To be more converging, the lens is made thicker by the action of the ciliary muscle surrounding it. The eye is most relaxed when viewing distant objects, one reason that microscopes and telescopes are designed to produce distant images. Vision of very distant objects is called totally relaxed, while close vision is termed accommodated, with the closest vision being fully accommodated.

Figure 3. Relaxed and accommodated vision for distant and close objects. (a) Light rays from the same point on a distant object must be nearly parallel while entering the eye and more easily converge to produce an image on the retina. (b) Light rays from a nearby object can diverge more and still enter the eye. A more powerful lens is needed to converge them on the retina than if they were parallel.

We will use the thin lens equations to examine image formation by the eye quantitatively. First, note the power of a lens is given as [latex]p=\frac{1}{f}\\[/latex], so we rewrite the thin lens equations as [latex]P=\frac{1}{d_{\text{o}}}+\frac{1}{d_{\text{i}}}\\[/latex] and [latex]\frac{h_{\text{i}}}{h_{\text{o}}}=-\frac{d_{\text{i}}}{d_{\text{o}}}=m\\[/latex].

We understand that di must equal the lens-to-retina distance to obtain clear vision, and that normal vision is possible for objects at distances do = 25 cm to infinity.

Take-Home Experiment: The Pupil

Look at the central transparent area of someone’s eye, the pupil, in normal room light. Estimate the diameter of the pupil. Now turn off the lights and darken the room. After a few minutes turn on the lights and promptly estimate the diameter of the pupil. What happens to the pupil as the eye adjusts to the room light? Explain your observations.

The eye can detect an impressive amount of detail, considering how small the image is on the retina. To get some idea of how small the image can be, consider the following example.

Example 1. Size of Image on Retina

What is the size of the image on the retina of a 1.20 × 10−2 cm diameter human hair, held at arm’s length (60.0 cm) away? Take the lens-to-retina distance to be 2.00 cm.

Strategy

We want to find the height of the image hi, given the height of the object is ho = 1.20 × 10−2 cm. We also know that the object is 60.0 cm away, so that do=60.0 cm. For clear vision, the image distance must equal the lens-to-retina distance, and so di = 2.00 cm . The equation [latex]\frac{h_{\text{i}}}{h_{\text{o}}}=-\frac{d_{\text{i}}}{d_{\text{o}}}=m\\[/latex] can be used to find hi with the known information.

Solution

The only unknown variable in the equation [latex]\frac{h_{\text{i}}}{h_{\text{o}}}=-\frac{d_{\text{i}}}{d_{\text{o}}}=m\\[/latex] is hi:

[latex]\displaystyle\frac{h_{\text{i}}}{h_{\text{o}}}=-\frac{d_{\text{i}}}{d_{\text{o}}}\\[/latex]

Rearranging to isolate hi yields

[latex]\displaystyle{h}_{\text{i}}=-h_{\text{o}}\cdot\frac{d_{\text{i}}}{d_{\text{o}}}\\[/latex].{-4}\text{ cm}\end{array}\\[/latex]

Discussion

This truly small image is not the smallest discernible—that is, the limit to visual acuity is even smaller than this. Limitations on visual acuity have to do with the wave properties of light and will be discussed in the next chapter. Some limitation is also due to the inherent anatomy of the eye and processing that occurs in our brain.

Example 2. Power Range of the Eye

Calculate the power of the eye when viewing objects at the greatest and smallest distances possible with normal vision, assuming a lens-to-retina distance of 2.00 cm (a typical value).

Strategy

For clear vision, the image must be on the retina, and so di = 2.00 cm here. For distant vision, do ≈ ∞, and for close vision, do = 25.0 cm, as discussed earlier. The equation [latex]P=\frac{1}{d_{\text{o}}}+\frac{1}{d_{\text{i}}}\\[/latex] as written just above, can be used directly to solve for P in both cases, since we know di and do. Power has units of diopters, where [latex]1\text{ D}=\frac{1}{\text{m}}\\[/latex], and so we should express all distances in meters.

Solution

For distant vision,

[latex]\displaystyle{P}=\frac{1}{d_{\text{o}}}+\frac{1}{d_{\text{i}}}=\frac{1}{\infty}+\frac{1}{0.0200\text{ m}}\\[/latex]

Since [latex]\frac{1}{\infty}=0\\[/latex], this gives [latex]P=0+\frac{50.0}{\text{m}}=50.0\text{ D}\\[/latex] (distant vision).

Now, for close vision,

[latex]\begin{array}{lll}P&=&\frac{1}{d_{\text{o}}}+\frac{1}{d_{\text{i}}}=\frac{1}{0.250\text{ m}}+\frac{1}{0.0200\text{ m}}\\\text{ }&=&\frac{4.00}{\text{m}}+\frac{50.0}{\text{m}}=4.00\text{ D}+50.0\text{ D}\\\text{ }&=&54.0\text{ D (close vision)}\end{array}\\[/latex]

Discussion

For an eye with this typical 2.00 cm lens-to-retina distance, the power of the eye ranges from 50.0 D (for distant totally relaxed vision) to 54.0 D (for close fully accommodated vision), which is an 8% increase. This increase in power for close vision is consistent with the preceding discussion and the ray tracing in Figure 3. An 8% ability to accommodate is considered normal but is typical for people who are about 40 years old. Younger people have greater accommodation ability, whereas older people gradually lose the ability to accommodate. When an optometrist identifies accommodation as a problem in elder people, it is most likely due to stiffening of the lens. The lens of the eye changes with age in ways that tend to preserve the ability to see distant objects clearly but do not allow the eye to accommodate for close vision, a condition called presbyopia (literally, elder eye). To correct this vision defect, we place a converging, positive power lens in front of the eye, such as found in reading glasses. Commonly available reading glasses are rated by their power in diopters, typically ranging from 1.0 to 3.5 D.

Section Summary

  • Image formation by the eye is adequately described by the thin lens equations:
    [latex]\displaystyle{P}=\frac{1}{{d}_{\text{o}}}+\frac{1}{{d}_{\text{i}}}\text{ and }\frac{{h}_{\text{i}}}{{h}_{\text{o}}}=-\frac{{d}_{\text{i}}}{{d}_{\text{o}}}=m\\[/latex].
  • The eye produces a real image on the retina by adjusting its focal length and power in a process called accommodation.
  • For close vision, the eye is fully accommodated and has its greatest power, whereas for distant vision, it is totally relaxed and has its smallest power.
  • The loss of the ability to accommodate with age is called presbyopia, which is corrected by the use of a converging lens to add power for close vision.

Conceptual Questions

  1. If the lens of a person’s eye is removed because of cataracts (as has been done since ancient times), why would you expect a spectacle lens of about 16 D to be prescribed?
  2. A cataract is cloudiness in the lens of the eye. Is light dispersed or diffused by it?
  3. When laser light is shone into a relaxed normal-vision eye to repair a tear by spot-welding the retina to the back of the eye, the rays entering the eye must be parallel. Why?
  4. How does the power of a dry contact lens compare with its power when resting on the tear layer of the eye? Explain.
  5. Why is your vision so blurry when you open your eyes while swimming under water? How does a face mask enable clear vision?

Problems & Exercises

Unless otherwise stated, the lens-to-retina distance is 2.00 cm.

  1. What is the power of the eye when viewing an object 50.0 cm away?
  2. Calculate the power of the eye when viewing an object 3.00 m away.
  3. (a) The print in many books averages 3.50 mm in height. How high is the image of the print on the retina when the book is held 30.0 cm from the eye? (b) Compare the size of the print to the sizes of rods and cones in the fovea and discuss the possible details observable in the letters. (The eye-brain system can perform better because of interconnections and higher order image processing.)
  4. Suppose a certain person’s visual acuity is such that he can see objects clearly that form an image 4.00 μm high on his retina. What is the maximum distance at which he can read the 75.0 cm high letters on the side of an airplane?
  5. People who do very detailed work close up, such as jewellers, often can see objects clearly at much closer distance than the normal 25 cm. (a) What is the power of the eyes of a woman who can see an object clearly at a distance of only 8.00 cm? (b) What is the size of an image of a 1.00 mm object, such as lettering inside a ring, held at this distance? (c) What would the size of the image be if the object were held at the normal 25.0 cm distance?

Glossary

accommodation: the ability of the eye to adjust its focal length is known as accommodation

presbyopia: a condition in which the lens of the eye becomes progressively unable to focus on objects close to the viewer

Selected Solutions to Problems & Exercises

1. 52.0 D

3. (a) −0.233 mm; (b) The size of the rods and the cones is smaller than the image height, so we can distinguish letters on a page.

5. (a) +62.5 D; (b) –0.250 mm; (c) –0.0800 mm

/ How your eye works (parts of the eye)Look After Your Eyes

Find out about different parts of the eye.

Basically, the role of the eye is to convert light into electrical signals called nerve impulses that the brain converts into images of our surroundings. Light rays pass through the pupil in the cornea.

Aqueous humour – maintains the pressure in your eye and nourishes the cornea and the lens by supplying amino acids and glucose, as well as vitamin C.

Choroid – a thin layer of blood vessels that nourish the retina and absorb scattered light.

Ciliary muscles – a circular muscle that relaxes or tightens to enable the lens to change shape for focusing.

Cornea – a clear covering on the front of your eye that focuses light entering the eye.

Fovea – a tiny pit in the macula that provides the sharp central vision that you need for activities, such as reading and driving.

Iris – the coloured part of your eye that regulates the amount of light entering.

Lens – the clear part of the eye behind the iris that helps to focus light, or an image, onto the retina.

Macula – the sensitive area in the centre of the retina responsible for what you see ahead of you (central vision).

Optic nerve – a bundle of more than one million nerve fibres that carries visual messages from the retina to the brain.

Pupil – this is the opening in the centre of the iris that lets in light. It is regulated by the iris.

Retina – the light-sensitive tissue lining at the back of your eye that converts light into electrical impulses that are sent along the optical nerve to the brain.

Sclera – also known as the white of your eye, this is the outer layer of the human eye.

Vitreous humour – a clear gel that fills the inside of the eye and helps it to retain its shape.

Tears

Tears have three main components; a watery component, an oily component and mucus. These create a film which covers the white of the eye and the cornea. A problem with any of the three layers may cause dry eye.

 

The structure and function of the eye, anatomy of the eye

A person sees not with his eyes, but through the eyes, from where information is transmitted through the optic nerve, chiasm, visual tracts to certain areas of the occipital lobes of the cerebral cortex, where the picture of the external world that we see is formed. All these organs make up our visual analyzer or visual system.

Having two eyes allows us to make our vision stereoscopic (that is, to form a three-dimensional image).The right side of the retina of each eye transmits through the optic nerve the “right side” of the image to the right side of the brain, similarly to the left side of the retina. Then the brain connects the two parts of the image – the right and the left – together.

Since each eye perceives “its own” picture, if the joint movement of the right and left eyes is disturbed, binocular vision can be impaired. Simply put, your eyes will start to see double or you will see two completely different pictures at the same time.

Basic functions of the eye

  • optical system for projecting an image;
  • a system that perceives and “encodes” the received information for the brain;
  • “service” life support system.

Eye structure

The eye can be called a complex optical device. Its main task is to “transmit” the correct image to the optic nerve.

The cornea is the transparent membrane that covers the front of the eye.There are no blood vessels in it, it has a great refractive power. It is included in the optical system of the eye. The cornea is bordered by the opaque outer shell of the eye – the sclera. See the structure of the cornea.

The anterior chamber of the eye is the space between the cornea and the iris. It is filled with intraocular fluid.

Iris – shaped like a circle with a hole inside (pupil). The iris is made up of muscles that, when contracted and relaxed, change the size of the pupil.It enters the choroid. The iris is responsible for the color of the eyes (if it is blue, it means that there are few pigment cells in it, if there are a lot of brown). Performs the same function as the aperture in a camera, adjusting the light flux.

Pupil – a hole in the iris. Its dimensions usually depend on the level of illumination. The more light, the smaller the pupil.

The lens is the “natural lens” of the eye. It is transparent, elastic – it can change its shape, almost instantly “directing focus”, due to which a person sees well both near and far.Placed in a capsule, held by by a ciliary band . The lens, like the cornea, is part of the optical system of the eye.

Vitreous body is a gel-like transparent substance located in the back of the eye. The vitreous body maintains the shape of the eyeball, participates in intraocular metabolism. It is included in the optical system of the eye.

Retina – consists of photoreceptors (they are sensitive to light) and nerve cells. The receptor cells located in the retina are divided into two types: cones and rods.In these cells, which produce the enzyme rhodopsin, the energy of light (photons) is converted into electrical energy of the nervous tissue, i.e., a photochemical reaction.

Sticks have a high light sensitivity and allow you to see in low light, they are also responsible for peripheral vision. Cones, on the contrary, require more light for their work, but it is they that make it possible to see small details (responsible for central vision), make it possible to distinguish colors.The largest accumulation of cones is located in the central fossa (macula), which is responsible for the highest visual acuity. The retina is adjacent to the choroid, but loose in many areas. It is here that it tends to flake off in various diseases of the retina.

Sclera – the opaque outer shell of the eyeball, passing in the front part of the eyeball into the transparent cornea. 6 oculomotor muscles are attached to the sclera. It contains a small number of nerve endings and blood vessels.

Choroid – lines the posterior part of the sclera, the retina is adjacent to it, with which it is closely connected. The choroid is responsible for the blood supply to the intraocular structures. In diseases of the retina, it is very often involved in the pathological process. There are no nerve endings in the choroid, therefore, with its disease, pain does not occur, usually signaling any malfunction.

Optic nerve – with the help of the optic nerve, signals from nerve endings are transmitted to the brain.

Useful to read

General questions about treatment at the clinic

Eye structure


Human eyes have a very complex structure. Opponents of evolution appeal precisely to the eye, saying that such a complex optical device could not develop “by itself.” The eye is part of the visual system, which also includes the optic nerve, chiasm and part of the lobes of the brain, where the light signal is processed and decoded into the picture that we see.

Human vision is three-dimensional thanks to two optical devices – the eyes. Each eye transmits its part of the image from the retina, which are then connected to a common image in the brain. That is why, if we close one or the other eye in turn, we see the central-axis displacement of the image. Double vision is called binocular disorder. It occurs if the eyes, for some reason, do not move synchronously.

General diagram of the structure of the eye


The cornea is the integumentary transparent membrane of the visible part of the eyeball.It is she who “reads” the light rays that carry visual information. The cornea has no vessels, it borders on the sclera – an opaque membrane.

The anterior chamber of the eye is a fluid-filled chamber between the cornea and the iris of the eye.

Iris is a colored, circle-like part of the choroid with a pupil in the middle. Responsible for adjusting the photo flux, is able to expand and narrow the pupil (bandwidth) depending on the brightness of the light.Consists of muscle tissue. The higher the pigmentation, the darker the eye color. In light-eyed people, the iris is almost unpigmented.

Pupil – located in the center of the iris, expanding or narrowing depending on the light. The more light, the narrower the pupils. Loss of pupil sensitivity is a sign of drug action or loss of vision.

Lens – a transparent elastic “lens” responsible for focusing the vision and clarity of the image reproduced by the brain.Held by an eyelash girdle. With age, it loses its transparency, which leads to cataracts. It can be replaced with an artificial one with full restoration of functions.

Vitreous body – consists of a gel-like substance, has a spherical shape, located in the posterior section. Supports intraocular pressure, has a compensatory function. Completely transparent, retransmits rays directly to the retina.

Retina – is responsible for converting light energy into electrical energy, which is transmitted through the nervous tissue to the brain.The process is provided by nerve cells and photoreceptors – rods and cones.

Rods – high sensitivity receptors. Thanks to them, we see at dusk and even in the dark. They also make it possible to see with “peripheral vision”.

Cones – detail receptors. They need good lighting to work, in return they allow us to distinguish the nuances of shades, to see small details. The most common retinal problem is retinal detachment. It arises due to the fact that there is initially no tight fit to the choroid.

Sclera – the back of the eye from the invisible side. Opaque, on the visible side passes into the cornea. It is to the sclera that the eye muscles are attached. The vessels and nerve endings feeding the eye are also concentrated here.

Choroid – the integumentary membrane of the sclera. The “station” of blood supply to the eye, the retina is adjacent to it. Due to the lack of nerve endings, it does not “signal” problems on its own. Therefore, if there is evidence, incl. hereditary diseases, injuries, it is necessary to see a doctor at least once a year.

Optic nerve – “wire” repeater of the received and decoded information to the brain. “Conductor” from the light beam in the visible picture.

Corneal structure

Epithelial layer is the protective layer of the cornea. Receives oxygen from the tear film, regulates eye moisture. Recovers if damaged.

Bowman’s membrane – the layer following the epithelial, incapable of renewal.Responsible for metabolic processes, nourishes the cornea.

Stroma is a volumetric layered part consisting of collagen fibers.

Descemet’s membrane is an elastic boundary membrane that separates the stroma from the endothelium.

Endothelium – regulator of moisture in the stratum corneum, poorly restored. Maintains the transparency of the cornea by “pumping” excess fluid.

With age, the density of the endothelium decreases by almost 3 times.From the initial 3500 cells per 1 sq. Mm. remains about 1500. If, as a result of traumatic lesions or inflammatory processes, the density of the endothelium falls below 800 l / 1 km.mm, corneal edema develops, vision rapidly falls.

Tear film – the so-called “sixth” layer of the cornea. The discharge of the lacrimal ducts has a sanitizing, softening, moisturizing value. Cleans from dust, increases transparency, facilitates oxygen absorption.


The eye as an optical system – a lesson.Physics, grade 8.

Most of the information that comes from the environment, a person receives through sight. The human eye is a complex and perfect optical system. Let’s take a look at how it works.

Fig. \(1\). The structure of the human eye

The fibrous membrane of the eyeball is the membrane of the eye that performs protective and shaping functions.

Sclera (from the Greek σκληρός – hard) – the outer white membrane of the eye, which protects from damage, the posterior part of the fibrous membrane.

The cornea is the most convex part of the eye, transparent light-refractive medium, the anterior part of the fibrous membrane.

Behind the cornea is the iris. In the iris, there is a round hole called the pupil. The iris can deform and thus change the diameter of the pupil. This change occurs reflexively (without the participation of consciousness), depending on the amount of light entering the eye. This property is called adaptation.

Adaptation – the ability of the eye to adapt to different brightness of the observed objects.

Inside the eye, directly behind the pupil, there is a lens, which is a transparent elastic body in the form of a biconvex lens. The curvature of the lens surfaces can change, thereby changing the optical power. This helps to regulate the distance from the lens to the image of the object, which should fall on the retina. The retina of the eye is its inner shell, which consists of branched nerve fibers and blood vessels.

Accommodation – the ability of the human eye to refract light rays in such a way as to see equally well at both close and medium and long distances.

The image obtained on the retina through the optic nerve enters the brain.

The vitreous body, a transparent gelatinous mass that fills the space between the lens and the retina, also takes part in the acquisition of the image. Light striking the surface of the eye is refracted in the cornea, lens and vitreous humor. As a result, a real, inverted, reduced image of the object is obtained on the retina.

Fig.\ (2 \). Image acquisition scheme

Sources:

Eye. https://www.shutterstock.com/ru/image-vector/human-body-parts-detailed-vector-set-134749994. 2021-08-22.

Fig. 1. The device of the human eye. Scheme. © YaKlass.

Fig. 2. The scheme of obtaining the image. Scheme. © YaKlass.

Lesson 1. How human vision works

Vision is a channel through which a person receives about 70% of all data about the world that surrounds him.And this is possible only for the reason that it is human vision that is one of the most complex and amazing visual systems on our planet. If there was no sight, we would all most likely just live in the dark.

The human eye has a perfect structure and provides vision not only in color, but also in three dimensions and with the highest sharpness. It has the ability to instantly change focus at a variety of distances, regulate the volume of incoming light, distinguish between a huge number of colors and even more shades, correct spherical and chromatic aberrations, etc.Six levels of the retina are connected to the brain of the eye, in which, even before the information is sent to the brain, the data goes through a compression stage.

But how does our vision work? How can we transform it into an image by enhancing the color reflected from objects? If you think about it seriously, we can conclude that the structure of the human visual system to the smallest detail is “thought out” by the Nature that created it. If you prefer to believe that the Creator or some Higher Power is responsible for the creation of man, then you can attribute this merit to them.But let’s not understand the secrets of life, but continue talking about the device of vision.

Huge number of parts

The structure of the eye and its physiology can be called really ideal. Think for yourself: both eyes are located in the bony cavities of the skull, which protect them from all kinds of damage, but they protrude from them just so that the widest possible horizontal view is provided.

The distance the eyes are from each other provides spatial depth.And the eyeballs themselves, as it is known for certain, have a spherical shape, due to which they are able to rotate in four directions: left, right, up and down. But each of us takes all this for granted – few people think of what it would be like if our eyes were square or triangular or their movement was chaotic – this would make vision limited, confused and ineffective.

So, the structure of the eye is extremely complex, but this is precisely what makes the work of about four dozen of its various components possible.And even if there were not even one of these elements, the process of vision would cease to be carried out the way it should be carried out.

To see how complex the eye is, we suggest you turn your attention to the figure below:

Let’s talk about how the process of visual perception is implemented in practice, what elements of the visual system are involved in this, and what each of them is responsible for.

Light transmission

As the light approaches the eye, the light rays collide with the cornea (otherwise it is called the cornea).The transparency of the cornea allows light to pass through it into the inner surface of the eye. Transparency, by the way, is the most important characteristic of the cornea, and it remains transparent due to the fact that a special protein that it contains inhibits the development of blood vessels – a process that occurs in almost every tissue of the human body. In the event that the cornea was not transparent, the rest of the components of the visual system would have no value.

Among other things, the cornea prevents debris, dust and any chemical elements from entering the inner cavities of the eye.And the curvature of the cornea allows it to refract light and help the lens focus the light rays on the retina.

After the light has passed through the cornea, it passes through a small hole located in the middle of the iris of the eye. The iris, on the other hand, is a circular diaphragm that sits in front of the lens just behind the cornea. The iris is also the element that gives the eye color, and the color depends on the pigment prevailing in the iris. The central hole in the iris is the pupil familiar to each of us.The size of this hole can be varied to control the amount of light entering the eye.

The size of the pupil will change directly by the iris, and this is due to its unique structure, because it consists of two different types of muscle tissue (even here there are muscles!) The first muscle is a circular contraction – it is located in the iris in a circular manner. When the light is bright, it contracts, as a result of which the pupil contracts, as if being pulled inward by the muscle.The second muscle is expanding – it is located radially, i.e. along the radius of the iris, which can be compared to the spokes in a wheel. In dark light, this second muscle contracts, and the iris opens the pupil.

Many evolutionary specialists still experience some difficulties when trying to explain how the above-mentioned elements of the human visual system are formed, after all, in any other intermediate form, i.e. at any evolutionary stage, they simply could not work, but a person sees from the very beginning of his existence.Riddle …

Focus

Bypassing the above stages, light begins to pass through the lens located behind the iris. The lens is an optical element in the shape of a convex oblong ball. The lens is absolutely smooth and transparent, there are no blood vessels in it, and it itself is located in an elastic sac.

Passing through the lens, light is refracted, after which it is focused on the retinal fossa – the most sensitive place containing the maximum number of photoreceptors:

It is important to note that the unique structure and composition provides the cornea and lens with high refractive power, guaranteeing a short focal length.And how amazing it is that such a complex system fits in just one eyeball (just think how a person would look if, for example, a meter were required to focus the light rays coming from objects!)

No less interesting is the fact that the combined refractive power of these two elements (cornea and lens) is in excellent correlation with the eyeball, and this can be safely called another proof that the visual system is created simply unsurpassed.

If we are talking about objects located close to the eye, then it is still more interesting here, because in this situation the refraction of light rays turns out to be even stronger. This is provided by an increase in the curvature of the lens. The lens is connected by means of ciliary bands to the ciliary muscle, which, by contracting, allows the lens to take on a more convex shape, thereby increasing its refractive power.

And here again it is impossible not to mention the complex structure of the lens: it is composed of many threads, which consist of cells connected to each other, and thin belts connect it with the ciliary body.Focusing is carried out under the control of the brain extremely quickly and on a full “automatic”, i.e. unconsciously.

The value of “photographic film”

Focusing results in the focusing of the image on the retina, which is a multi-layered tissue sensitive to light that covers the back of the eyeball. The retina contains about 130 million photoreceptors (for comparison, modern digital cameras can be cited, in which there are no more than 10,000,000 such sensor elements) [Kumaramanickavel G., Denton M. J., Legge M., 2015]. Such a huge number of photoreceptors is due to the fact that they are located extremely tightly – about 400,000 per 1 mm².

Here it will not be superfluous to quote the words of the microbiologist Alan L. Gillen, who speaks in his book “The Body by Design” about the retina of the eye as a masterpiece of engineering design. He believes that the retina is the most amazing element of the eye, comparable to photographic film. The light-sensitive retina, located on the back of the eyeball, is much thinner than cellophane (its thickness is no more than 0.2 mm) and much more sensitive than any human-made photographic film.The cells of this unique layer are capable of processing up to 10 billion photons, while the most sensitive camera can process only a few thousand of them [Gillen A. L., 2001]. But even more surprising is that the human eye can pick up a few photons even in the dark:

In total, the retina consists of 10 layers of photoreceptor cells, 6 of which are layers of light-sensitive cells. The 2 types of photoreceptors have a special shape, which is why they are called cones and rods.The rods are extremely sensitive to light and provide black-and-white perception and night vision to the eye. Cones, in turn, are not so sensitive to light, but they are able to distinguish colors – the optimal operation of the cones is noted during the daytime.

Thanks to the work of photoreceptors, light rays are transformed into complexes of electrical impulses and are sent to the brain at an incredibly high speed, and these impulses themselves overcome over a million nerve fibers in a fraction of a second.

The communication of photoreceptor cells in the retina is very complex.The cones and rods are not directly connected to the brain. Having received the signal, they redirect it to the bipolar cells, and they redirect the signals already processed by themselves to the ganglion cells, more than a million axons (neurites through which nerve impulses are transmitted) of which make up a single optic nerve, through which the data goes to the brain:

Two layers of intermediate neurons, before visual data are sent to the brain, facilitate the parallel processing of this information by six levels of perception located in the retina.This is necessary in order for images to be recognized as quickly as possible.

Brain Perception

After the processed visual information enters the brain, it begins sorting, processing and analyzing it, and also forms a whole image from the individual data. Of course, a lot is still unknown about the work of the human brain, but even what the scientific world can provide today is quite enough to be amazed.

With the help of two eyes, two “pictures” of the world that surrounds a person are formed – one for each retina.Both “pictures” are transmitted to the brain, and in reality a person sees two images at the same time. But how?

And the thing is this: the point of the retina of one eye exactly corresponds to the point of the retina of the other, and this suggests that both images, entering the brain, can be superimposed on each other and combined together to obtain a single image. The information received by the photoreceptors of each of the eyes converges in the visual cortex of the brain, where a single image appears.

Due to the fact that two eyes can have different projections, some inconsistencies can be observed, but the brain compares and connects the images in such a way that the person does not feel any inconsistencies. Moreover, these discrepancies can be used to gain a sense of spatial depth.

As you know, due to the refraction of light, the visual images entering the brain are initially very small and inverted, but “at the output” we get the image that we are used to seeing.

In addition, in the retina, the image is divided in two by the brain vertically – through a line that passes through the retinal fossa. The left sides of the images taken with both eyes are redirected to the right hemisphere, and the right sides to the left. Thus, each of the hemispheres of the observing person receives data from only one part of what he sees. And again – “at the output” we get a solid image without any trace of the connection.

Image separation and highly complex optical pathways make the brain see each of its hemispheres separately using each of the eyes.This allows you to speed up the processing of the flow of incoming information, and also provides vision with one eye, if suddenly a person for some reason ceases to see the other.

It can be concluded that the brain in the process of processing visual information removes “blind” spots, distortions due to micromovements of the eyes, blinking, angle of view, etc., offering its owner an adequate integral image of the observed.

Eye movement

Another important element of the visual system is eye movement.There is no way to belittle the significance of this question, since in order to be able to use vision properly, we must be able to turn our eyes, raise them, lower them, in short, move our eyes.

In total, 6 external muscles can be distinguished, which are connected to the outer surface of the eyeball. These muscles include 4 straight (lower, upper, lateral and middle) and 2 oblique (lower and upper):

At the moment when any of the muscles contracts, the muscle, which is opposite to it, relaxes – this ensures even movement of the eyes (otherwise all eye movements would be carried out in jerks).

Turning two eyes automatically changes the movement of all 12 muscles (6 muscles for each eye). And it is noteworthy that this process is continuous and very well coordinated.

Control and coordination of the communication of organs and tissues with the central nervous system through the nerves (this is called innervation) of all 12 eye muscles is one of the very complex processes occurring in the brain. If we add to this the accuracy of redirecting the gaze, the smoothness and evenness of movements, the speed with which the eye can rotate (and it adds up to 700 ° per second), and combine all this, we will actually get a phenomenal in terms of performance movable eye system.And the fact that a person has two eyes makes it even more difficult – with the synchronous movement of the eyes, the same muscular innervation is necessary.

The muscles that rotate the eyes are different from the muscles of the skeleton. they are made up of many different fibers, and they are controlled by an even larger number of neurons, otherwise the accuracy of movements would become impossible. These muscles can be called unique also because they are able to quickly contract and practically do not get tired.

Eye Cleaning

Considering that the eye is one of the most important organs of the human body, it needs continuous care.It is precisely for this that the “integrated cleaning system”, which consists of eyebrows, eyelids, eyelashes and lacrimal glands, is provided, if you can call it that:

With the help of the lacrimal glands, a sticky liquid is regularly produced, moving at a slow speed down the outer surface of the eyeball. This liquid washes away various debris (dust, etc.) from the cornea, after which it enters the internal lacrimal canal and then flows down the nasal canal, being excreted from the body.

Tears contain a very strong antibacterial substance that destroys viruses and bacteria.The eyelids function as wipers – they cleanse and moisturize the eyes through involuntary blinking at intervals of 10-15 seconds. Together with the eyelids, eyelashes also work, preventing any debris, dirt, microbes, etc. from entering the eye.

If the eyelids did not fulfill their function, the human eyes would gradually dry out and become covered with scars. If there was no tear duct, the eyes would be constantly filled with tear fluid. If the person did not blink, debris would fall into his eyes, and he might even go blind.The entire “cleaning system” must include the work of all elements without exception, otherwise it would simply cease to function.

Eyes as an indicator of condition

Human eyes are capable of transmitting a lot of information in the process of his interaction with other people and the world around him. The eyes can radiate love, burn with anger, reflect joy, fear or anxiety, and speak of anxiety or fatigue. The eyes show where a person is looking, whether he is interested in something or not.

For example, when people roll their eyes while talking to someone, this can be interpreted in a completely different way from the usual upward gaze. Big eyes in children cause delight and affection in those around them. And the state of the pupils reflects the state of consciousness in which a person is at a given moment in time. Eyes are an indicator of life and death, if we speak in a global sense. Probably for this reason they are called the mirror of the soul.

Instead of a conclusion

In this lesson, we examined the structure of the human visual system.Naturally, we missed a lot of details (this topic itself is very voluminous and it is problematic to fit it into the framework of one lesson), but still we tried to convey the material so that you get a general idea of ​​HOW a person sees.

You could not help but notice that both the complexity and the capabilities of the eye allow this organ to be many times superior to even the most modern technologies and scientific developments. The eye is a clear demonstration of the complexity of engineering in a huge number of nuances.

But knowing about the vision device is, of course, good and useful, but the most important thing is to know how vision can be restored. The fact is that a person’s lifestyle, and the conditions in which he lives, and some other factors (stress, genetics, bad habits, diseases, and much more) – all this often contributes to the fact that over the years vision can deteriorate, i.e. .e. the visual system starts to malfunction.

But the deterioration of vision in most cases is not an irreversible process – knowing certain methods, this process can be reversed, and vision, if not the same as that of a baby, then as good as it is generally possible for each individual person.

Look at the root!

Test your knowledge

If you want to test your knowledge of the topic of this lesson, you can take a short test consisting of several questions. In each question, there can be only one correct answer. After you have selected one of the options, the system automatically proceeds to the next question. The points you receive are influenced by the correctness of your answers and the time spent on passing. Please note that the questions are different each time, and the options are mixed.

The next lesson is devoted to methods of restoring vision.

Kirill Nogales

Eye structure

The human eye is a complex organ in which all many parts work wonderfully harmoniously. The work of the brain is even more striking. After all, it is he who controls the eyes, forcing them to instantly respond to different lighting, focus on focus, detect objects and track their slightest movements, build a three-dimensional picture of the surrounding world – and all this is only based on a two-dimensional kaleidoscope of light spots on the retina! That is why the usual comparison of the eye with the camera is incorrect.Of course, the camera largely repeats the principles of the eye, and we will use this convenient analogy more than once. But in reality, the eye is more like an automatic CCTV camera controlled by a super-powerful computer – the brain.

Many animals got a visual “device” much better than ours. You have probably seen in the optometrists’ offices the Snellen optotype chart, which is used to determine visual acuity. So, some birds of prey can easily distinguish the bottom line of this table from a distance of more than 30 meters.In the retina of butterflies, there are up to 8 types of cones, which gives them such a quality of color perception that we could not even dream of with 3 types! The eyes of many insects move much faster than ours, and they are arranged in a completely different way.

But there is no such eye in nature, all characteristics of which would be equally good. The development of one or another of them depends on environmental conditions and on the tasks facing a living being. And the human eye is a wonderful device, despite all the shortcomings that will be discussed later.

An “autofocus lens” consisting of two lenses – a fixed cornea and an elastic lens focuses images of surrounding objects on the retina (“film”, or, if you like, “digital matrix”), smoothly following moving objects and compensating for head movements (optical stabilization system “). The pupil, the living “diaphragm” of our “lens”, reacts to light levels by contracting and expanding. The maximum value of this “aperture” is approximately 3.5, that is, the lens we got is not the fastest.The retina converts light into chemical energy, which in turn is converted into a nerve signal to the brain (Figure 1).

Fig. 1. Human eye in section: the main units of our “optical device”.

Unlike flat photographic films and matrices, the retina is hemispherical. Thanks to this design, light falls on sensitive elements at different angles, and the illusion of depth of space is recreated in the brain (binocular vision is also responsible for the illusion of volume of individual objects, which causes a stereoscopic effect).

Another important feature of the retina is the constant adaptation to the light level, allowing it to capture many shades of brightness. No other photographic film has such a dynamic range, and digital sensors are still far from film.

The most sensitive area of ​​the retina is the macula (macula) and especially its central fossa (fovea). It is here that in the emmetrope, that is, a person with healthy refraction, the light rays collected by the cornea and lens are focused. It is the macula that is responsible for clear central vision, but it occupies only a small fraction of the retinal area: the diameter of the macula is 5 mm, the diameter of the central fossa is 1.5 mm.Therefore, to cover the entire field of view, the eye must constantly perform microscopic scanning movements – saccades. These movements are imperceptible, involuntary, lightning fast; they happen even if you meditate on the flame of a candle and think that your gaze is completely still. Each saccade is an attempt by the eye to grasp another fragment from the complex mosaic of the world. The brain has to “glue” this coherent mosaic of continuous visual data in real time, so that the process of perception is incredibly complex and does not boil down to “photographing”.Eyes give people 75% of the information about the world, but it comes at a high price. The load on the sensory system is so great that nature does not hesitate to sacrifice vision if it can be dispensed with. Look at the blind cave fish, accompanied by its closest relatives living near the surface of the water (Fig. 2).

Fig. 2. A blind cave fish next to two fish of the same species constantly living near the surface of the water.

The popular modern joke that life is a game with a mediocre design but stunning graphics doesn’t lack real meaning at all. It is not entirely correct to convert the analog signal of the eye into digital megapixels, although in principle it is possible, and we will definitely deal with this when we talk about the retina in more detail. To begin with, I would like to stun the reader with an approximate final figure: 576 megabytes! This is how much the final “file” collected by the brain weighs, which we perceive as a picture of the surrounding world.Imagine how even a powerful computer will slow down when processing such a file in a graphics editor. But we are jokingly doing this every moment! The better the “graphics” (visual perception), the better the “iron” (nervous system and brain) should be, the more the body’s forces are consumed. A person spends more than 50% of all energy obtained from food on processing visual information! That is why we feel so tired after sitting all day at the computer or going around all the halls of a large museum.

The durability and reliability of any device depends not only on how well it is designed, but also on the operating conditions. To be honest, we were out of luck with the conditions. And the main problem is not even a spoiled ecology or improper nutrition. The fact is that almost all the time we use our eyes in a different mode of operation for which evolution has prepared us for millions of years. The human eye is simply not designed to read small icons from a distance of 30 cm most of the time, especially from a monitor screen glowing with its own light.There is no such light in nature – there is only the light of the sun reflected from objects, light with a completely different, warm spectrum.

That is why fluorescent lamps with a cold, bluish tint are very harmful to the eyes, primarily to the retina. And the LCD monitor is doubly harmful – almost the same fluorescent lamp shining directly into your face! Yes, imagine, contrary to advertising myths and the general direction of development of production, CRT monitors were not only better in terms of picture and color reproduction (many designers still prefer to use them), but also safer for the eyes.By the way, the obsessive advertising of miraculous means of “protection from monitor radiation” is simply lying. In principle, there is no protection against electromagnetic radiation. The cold light of the monitor itself is harmful to the eyes, and this can only be corrected with the help of special optical filters-glasses. It is not easy to produce such glasses; this requires serious technologies that are available only to serious companies and organizations. “Fedorov glasses” advertised on every corner have nothing to do with Academician S.Fedorov and his institute, nor to protect your eyes.

Why has childhood myopia become an epidemic in recent decades? Because, according to the design of nature, a child should play and run in the fresh air, and not stare for hours at a textbook or at a monitor screen. It is not for nothing that almost all of us in early childhood are naturally a little farsighted. Gradually, the eyeball grows, and this imbalance is corrected, but many are unlucky: myopia develops … What if in childhood you cannot refuse to study, in adulthood – from working at a computer, which means from an unnatural load on the eyes? In this case, answer the age-old question “What to do?” on two pages is impossible – the entire section will be devoted to this.First, let’s try to decide what NOT to do. This question can be answered much more simply and unambiguously. Do not allow long visual work near without rest for at least 10 minutes per hour, moreover, active rest: look from a book or monitor to more distant objects in the room, outside the window. If the child develops myopia, do not try to train accommodation by drawing dots on the window pane: this will only reinforce the pathology. Don’t neglect eye gymnastics, but be wary of recommendations from books with cute titles like Killer Glasses.

Timely and correctly selected optical correction in childhood is very important to combat progressive myopia. There is nothing good in the fact that a myopic child without glasses or with undercorrection (the dubious legacy of the Soviet ophthalmological school) has to constantly strain his eyes, looking at the blackboard. It is absolutely precisely established that myopia grows from this! Do not rush to seek help from surgery: this is a last resort, associated with many side effects and is not necessary in all cases.Do not buy “protective screens” for the monitor or “Fedorov glasses for working at the computer.” “Protective screens” will only worsen the image and make you strain your eyes even more, and the so-called “Fedorov glasses” are not made at the MNTK, but somewhere on Malaya Arnautskaya.

By following these simple anti-recommendations, you can save your eyes until we meet again!

Genesis Vision Center “35 facts about eyes you did not know

26 September 2019

We recommend all office workers to think more often about the state of vision and at least sometimes do exercises for the eyes.

  1. The pupils of the eyes dilate by almost half when we look at the one we love.
  2. The cornea of ​​the human eye is so similar to the cornea of ​​a shark that the latter is used as a substitute for eye surgeries.
  3. Each eye contains 107 million cells, all of which are sensitive to light.
  4. Every 12th male is color blind.
  5. The human eye can perceive only three parts of the spectrum: red, blue and yellow. The rest of the colors are a combination of these colors.
  6. Our eyes are about 2.5 cm in diameter and weigh about 8 grams.
  7. Only 1/6 of the eyeball is visible.
  8. On average, over a lifetime, we see about 24 million different images.
  9. Your fingerprints have 40 unique characteristics, while the iris has 256. It is for this reason that retinal scans are used for security purposes.
  10. People say “you can’t blink an eye” because it is the fastest muscle in the body.Blinking lasts about 100 – 150 milliseconds, and you can blink 5 times per second.
  11. The eyes transmit a huge amount of information to the brain every hour. The bandwidth of this channel is comparable to that of Internet providers in a large city.
  12. The brown eyes are actually blue underneath the brown pigment. There is even a laser procedure that can turn brown eyes blue forever.
  13. Our eyes focus on about 50 things per second.
  14. The images that are sent to our brains are actually inverted.
  15. Eyes work the brain more than any other part of the body.
  16. Each eyelash lasts about 5 months.
  17. The Maya found squint attractive and tried to make their children squint.
  18. About 10,000 years ago, all people had brown eyes, until a person living in the Black Sea region developed a genetic mutation that led to blue eyes.
  19. If you have only one red eye in a flash photo, chances are that you have eye swelling (in case both eyes are looking in the same direction at the camera).Fortunately, the cure rate is 95%.
  20. Schizophrenia can be determined with an accuracy of 98.3% using a routine eye movement test.
  21. Humans and dogs are the only ones looking for visual cues in the eyes of others, and dogs only do this when they interact with humans.
  22. About 2% of women have a rare genetic mutation that causes them to have an extra retinal cone. This allows them to see 100 million colors.
  23. Johnny Depp is blind in his left eye and myopia in his right.
  24. Recorded a case of Siamese twins from Canada with a common thalamus. Thanks to this, they could hear each other’s thoughts and see each other’s eyes.
  25. The human eye can make smooth (non-intermittent) movements only if it is following a moving object.
  26. The history of the Cyclops originated from the peoples of the Mediterranean islands, who discovered the remains of extinct dwarf elephants. The skull of elephants was twice the size of a human skull, and the central nasal cavity was often mistaken for the orbit.
  27. Astronauts cannot cry in space due to gravity. Tears collect in small balls and begin to pinch eyes
  28. The pirates used a blindfold to quickly adapt their vision to the environment above and below deck. Thus, one eye became accustomed to bright light, and the other to dim light.
  29. There are colors that are too “difficult” for the human eye, they are called “impossible colors”. For example, red-green color.
  30. We see certain colors because this is the only spectrum of light that passes through the water – the area where our eyes appeared.There was no evolutionary reason on earth to see a broader spectrum.
  31. Eyes began to evolve about 550 million years ago. The simplest eye was particles of photoreceptor proteins in unicellular animals.
  32. Occasionally, people with aphakia, the lack of a lens, report seeing the ultraviolet spectrum of light.
  33. Bees have hairs in their eyes. They help determine wind direction and flight speed.
  34. Apollo astronauts reported seeing flashes and streaks of light when they closed their eyes.It was later revealed that this was caused by cosmic radiation irradiating their retinas outside the Earth’s magnetosphere.
  35. We “see” with our brain, not our eyes. Blurry and poor quality image is a disease of the eyes, as a sensor receiving an image with distortion. Then the brain will impose its distortions and “dead zones”.

What is the Optical system of the eye

From the point of view of physical optics, the human eye is referred to as centered
optical systems, which are characterized by the presence of 2 or more lenses, which
have one common main optical axis.

The optical system of the eye is the optical apparatus of the eye, which includes
living lenses (lens and cornea, between which there is a diaphragm),
vitreous and aqueous humor. It also includes lacrimal fluid,
providing transparency of the cornea. Basic refractive surfaces
of this system – these are both surfaces of the lens and the anterior surface of the cornea.
The function of the rest of the media is mainly to conduct light.

The eye perceives the objects of the external world under consideration, analyzing them
images on the retina. Functionally, the eye is divided into 2 key
department: light-perceiving and light-guiding.

The light-conducting section includes the transparent media of the eye: the cornea,
moisture in the anterior chamber, vitreous humor and lens. Light-receiving department
Is the retina. With the help of an optical system of light-guiding media, the image
objects are reproduced on the retina.

Reflecting from the objects under consideration, the rays of light pass through 4
refractive surfaces: posterior and anterior surfaces of the cornea, posterior
and the anterior surface of the lens. Passing through each of them, the beam is deflected
from the initial direction, as a result, at the focus of the optical system, we get
real, but inverted by 180 degrees, the image of the object on which
look. There is such a thing as refraction, which means the refraction of light.
in the optical system.

The optical axis of the eye is a straight line that passes through the centers
curvatures of each of the refracting surfaces. The rays of light that fall
parallel to a given axis, after refraction, are joined together at the main focus
systems. Parallel rays come from infinitely distant objects, and the main
the focus of the optical system is a place on the extension of the optical axis, in
which forms the image of objects that are infinitely distant.

Diverging rays that come from objects on any particular
distance will gather in additional focuses. They will be located
farther than the main focus, because to focus the diverging rays, you need
additional refractive power, and the stronger the divergence of the incident rays, the
it must be greater, i.e., it increases with the approach of the source of these
rays.

The distance between the main plane and the main focus is the main focal point.
distance of the optical system.

The optical power of the system depends on the focal length. The shorter it is, the
the system refracts more strongly. The optical power of lenses is measured using the value,
which is the reciprocal of the focal length, called the diopter.

One diopter (diopter) is the refractive power of the lens at focal length
one meter. Knowing the focal length of the lens, you can determine its refraction.

To fully characterize the optical system of the eye, you need to know
the radii of curvature, both of the anterior and posterior surfaces of the cornea, and
lens, as well as the thickness of the lens and cornea, determine the length
anatomical axis of the eye, anterior chamber depth and key indicators
refraction of transparent media.