eye innards

Created for VSP by the Exploratorium

2 | www.exploratorium.edu

Created for VSP by the Exploratorium®

Introduction ................................................................................................................................................................. 3

PART 1: EYE INNARDS ................................................................................................................4Look into Your Eyes .................................................................................................................................................... 5Animal Eyes .................................................................................................................................................................. 7Cow’s Eye Dissection ................................................................................................................................................. 8Glossary........................................................................................................................................................................11Exercise Your Eyes .....................................................................................................................................................12Lights On, Lights O!: Catch your pupils at work...........................................................................................13Eyedenti"cation: Iris Scanning ............................................................................................................................14See the Blood Vessels in Your Eye: Use a penlight to explore your eye ................................................15Find Your Blind Spot: A disappearing dot points the way to your blind spot ....................................16

PART 2: SEEING STUFF .............................................................................................................17TV Projector: Use a lens to project a TV image onto paper .......................................................................18Coloring in the Dark: Turn out the lights and pick your colors—can it be done? .............................19Pirate Patch: Cover one eye—and see better in the dark ..........................................................................20How to See in the Dark ...........................................................................................................................................21Hole in Your Hand: See through your hand using the power of stereo vision ...................................22Hot Dog Finger: Float an extra "nger in front of your face .......................................................................23The Language of the Eyes .....................................................................................................................................24Flipsticks: Make cartoon drawings into moving pictures .........................................................................25How Do Movies Work? ............................................................................................................................................28Fading Dot: The harder you stare, the less you’ll see ...................................................................................29How Your Eyes Move ...............................................................................................................................................30Find Your Dominant Eye: Which of your eyes has the power? ................................................................31Afterimages: Can you put a bird in the cage, just by staring at it? .........................................................32When Things Go Wrong .........................................................................................................................................34

PART 3: LOOKING THROUGH LENSES .....................................................................................36Using Lenses to Change How Things Look .....................................................................................................37How Lenses Help You See......................................................................................................................................38Waterdrop Lenses: Magnify the morning paper with a few drops of water .......................................39Early Eyeglasses ........................................................................................................................................................40Don’t Forget Your Sunglasses! .............................................................................................................................41Breaking Straws: Use a glass of water to break a straw in two .................................................................42Finding Re#ections: Take a look at what’s looking back ............................................................................43Accidental Lenses: Finding new ways to look at things .............................................................................44Log On to Good Computer Habits .....................................................................................................................45

PART 4: FOOLING YOUR EYES & BRAIN ..................................................................................46Hat Trick: Tape a mirror to your hat—and forget how to write your own name ..............................47Colored Words: How fast can you name these colors? ...............................................................................48Whirling Colors: What’s black and white—and spins into color? ...........................................................49Seeing Faces Everywhere: Are you looking at me? .....................................................................................51The Man in the Moon May be a Rabbit ............................................................................................................52Arc Illusion: Here’s the short and long of false comparisons ....................................................................53Dime Illusion: How do the magic circles make a dime shrink and grow? ............................................54The Amazing Shrinking Person: The farther you go, the smaller you grow ........................................55

Acknowledgments and Credits ...........................................................................................................................57



Eyecare Discovery Activity Guide: A set of activities exploring the eye for learners of all ages

Welcome to the Eyecare Discovery Activity Guide developed by the Exploratorium and VSP Vision Care for little kids, bigger kids, and anyone who wants to explore how eyes work. We tried to make all the activities hands-on, using easy-to-"nd supplies, so explorers can learn by doing as they investigate and experiment. We hope these activities will inspire our readers’ curiosity so they carry their eye explorations further.

The Eyecare Discovery Activity Guide is divided into four parts:

1. Eye Innards—all the di!erent parts of the eye;

2. Seeing Stu!—how light creates an image in the brain and how rods and cones work;

3. Looking Through Lenses—how lenses, both inside and outside our eyes, change the way we see things; and

4. Fooling Your Eyes—how illusions work and what they tell us about the eye/brain connection.

Each section can be used independently of the others, and each contains supplemental material about eyecare and safety as well as interesting eye information.



Poets and artists may see life, love, and laughter in a person’s eyes, but what’s really going on in there?

Your eyes are remarkable instruments, designed, essentially, to gather light, organize and encode it, and send the information your brain needs to create a visual picture of the world around you.

The information and activities in this section will introduce you to the parts of the eye and how they work. You’ll "nd the point in your eye where things can disappear, watch the iris of your eye open to let light in, see the shadows of blood vessels inside your eye, look inside a cow’s eye, "nd out how to exercise your eyes, and more.



Gaze into a mirror and what do you see? Your eyes of course!

Most of your eye is nestled inside the bony protection of your skull, but there’s a lot you can see on the outside, too.

This is the colored part of your eye. All irises have the same dark pigment, but darker-colored eyes have more of it than lighter-colored eyes. The iris is made of a circular, radiating muscle that opens and closes to control the light that gets in through your pupil.

A hole in the center of your iris where the light can get into your eye.

Tiny holes at the top and bottom of your eyelid near your nose. You’re always releasing tears to lubricate your eyes. These holes act as drains for excess lubrication. When you cry, these holes over#ow and the tears run down your cheeks. You may have to gently pull down your eyelid to see this.

Close your eyes and press gently on your eyelid. You’ll be able to feel the slight bulge of the cornea in the middle of your eye. It’s a tough, clear window that protects your iris and pupil.

Blinking both lubricates and protects your eyes. Most people blink 12 to 15 times a minute. You can make yourself blink any time you think of it, but blinking when something comes #ying at you is a re#exive action that protects your eyes.

Your hairy eyebrows and eyelashes help protect your eyes by catching bits of dust, pollen, and even skin cells—any foreign matter that might "nd its way into your eyes.

The thick outer covering of your eyeball; the “whites” of your eyes.

Tiny arteries and veins carry blood to all parts of your eye, and you can see some here. Irritation from rubbing or crying or lack of sleep can make them stand out and your eyes look red.



The colorful, shiny material located behind the retina. Found only in animals with good night vision, the tapetum re#ects light back through the retina. You don’t have a tapetum, but dogs, cats, and cows do.

A tough, clear covering over the iris and the pupil that helps protect the eye. Light bends as it passes through the cornea. This is the "rst step in making an image on the retina. The cornea begins bending light to make an image; the lens "nishes the job.

The pupil is the dark circle in the center of your iris. It’s the hole that lets light into the inner eye. Your pupil is round. A cow’s pupil is oval.

A clear #uid that helps the cornea keep its rounded shape.

A muscle that controls how much light enters the eye. It is suspended between the cornea and the lens. A cow’s iris is brown. Human irises come in many colors, including brown, blue, green, and gray.

A clear, #exible structure that makes an image on the eye’s retina. The lens is #exible so that it can change shape, focusing on objects that are close up and objects that are far away. The thick, clear jelly that helps give

the eyeball its shape.

The thick, tough, white outer covering of the eyeball.

The layer of light-sensitive cells at the back of the eye. The retina detects images focused by the cornea and the lens. The retina is connected to the brain by the optic nerve.

The place where the optic nerve leaves the retina. Each eye has a blind spot where there are no light-sensitive cells.

The bundle of nerve "bers that carries information from the retina to the brain.

Most of your eye is hidden inside your head. That’s a good thing, since the bones of your skull protect the complex and fragile inner workings of the eye.

If you could look inside an eye, here are the parts you would see:



Quick, what’s the di!erence between your eyes and the eyes of a gecko? A cat? An owl? A kangaroo? Read on to learn how very di!erent eyes can be!

• Some geckos have eyes with pupils shaped like slits. Why aren’t they round, like the pupils in your eyes? These lizards hunt at night, so they have good night vision. If they had round pupils, bright sunlight during the day would overwhelm their eyes, which are very sensitive to light. In bright light, the gecko’s iris squeezes shut so the pupil looks as narrow as the hands of a clock at 6:00 p.m.

• Cats also have good night vision. In bright light, a cat’s pupil shuts down to a slit. And like some other animals (including dogs, cows, and sheep), cats have a tapetum, a layer of light-re#ective cells that lines their eyes. Light bouncing o! this re#ective layer makes your cat’s eyes shine in the dark.

• Owls hunt at night, too, and they have very large eyes

to help them see well in the dark. But unlike our eyes, an owl’s eyes are "xed in place, staring straight ahead. To look in a di!erent direction, an owl must turn its head. Owls are so good at this they can turn their heads to look behind them.

• Goats, kangaroos, and antelope have rectangular pupils that are horizontal to the ground. These animals keep a sharp eye out for predators, and rectangular pupils let them watch over a large area.

• Crabs have eyes on stalks that stick up above their head, letting them look in many di!erent directions without turning around.

• Animals such as rabbits and deer have eyes on the sides of their head, with very little overlap in each eye’s visual "eld. This kind of vision, called “all-around vision,” lets them scan large areas both to the right and the left, which helps them quickly spot predators.



One way to "gure out how something works is to look inside it. To learn about how your eyes work, you can dissect, or take apart, a cow’s eye.

You’ll be using a scalpel or a razor to cut the cow’s eye. Be careful. A scalpel or razor can cut you as easily as it cuts the cow’s eye.

Whenever you handle raw meat, whether it’s a cow’s eye or a steak, wash your hands thoroughly afterward to wash away any bacteria you picked up from the meat. If you have cuts on your hands, you should also wear gloves so that no bacteria from the cow’s eye infects your cut.

• One cow’s eye for every two participants• One single-edged razor blade or scalpel for

every two participants• Scissors (optional)• Wax paper and paper towels• Plastic garbage bag• A cutting board• A sheet of newspaper• Soap, water, and paper towels for cleaning up

Here’s where to get cows’ eyesYou can order cows’ eyes at a butcher shop or purchase them directly from a slaughterhouse. Try to get eyes with the muscles and fat still attached. If possible, pick up the cows’ eyes the day of the dissection; eyes are easier to cut when they are fresh.

1. Examine the outside of the eye. See how many parts of the eye you can identify. You should be able to "nd the whites (or sclera), the tough, outer covering of the eyeball. You should also be able to identify the fat and muscle surrounding the eye. You should be able to "nd the rounded covering over the front of the eye (the cornea). When the cow was alive, the cornea was clear. In your cow’s eye, the cornea may be cloudy. You may be able to look through the cornea and see the iris, the colored part of the eye, and the pupil, the dark oval in the middle of the iris.

2. Cut away the fat and muscle.

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3. Use a scalpel to make an incision in the cornea. (Care-ful—Don’t cut yourself!) Cut until the clear liquid under the cornea is released. That clear liquid is the aqueous humor. It’s made mostly of water and keeps the shape of the cornea.

4. Use the scalpel to make an incision through the sclera in the middle of the eye.

5. Use your scissors to cut around the middle of the eye, cutting the eye in half. You’ll end up with two halves. On the front half will be the cornea.

The cornea is made of pretty tough stu!—it helps protect your eye. It also helps you see by bending the light that comes into your eye.

Once you have removed the cornea, place it on the board or cutting surface and cut it with your scalpel or razor. Listen. Hear the crunch? That’s the sound of the scal-pel crunching through layers of clear tissue. The cow’s cornea has many layers to make it thick and strong. When the cow is grazing, blades of grass may poke the cow’s eye—but the cornea protects the inner eye.

6. The next step is to pull out the iris. The iris is between the cornea and the lens. It may be stuck to the cornea or it may have stayed with the back of the eye. Find the iris and pull it out. It should come out in one piece. You can see that there’s a hole in the center of the iris. That’s the pupil, the hole that lets light into the eye. The iris con-tracts or expands to change the size of the pupil. In dim light, the pupil opens wide to let light in. In bright light, the pupil shuts down to block light out.

7. The back of the eye is "lled with a clear jelly. That’s the vitreous humor, a mixture of protein and water. It’s clear so light can pass through it. It also helps the eyeball maintain its shape.

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Now you want to remove the lens. It’s a clear lump about the size and shape of a squashed marble.

8. The lens of the cow’s eye feels soft on the outside and hard in the middle. Hold the lens up and look through it. What do you see?

9. Put the lens down on a newspaper and look through to the words on the page. What do you see?

10. Now take a look at the rest of the eye. If the vitreous humor is still in the eyeball, empty it out. On the inside of the back half of the eyeball, you can see some blood vessels that are part of a thin #eshy "lm. That "lm is the retina. Before you cut the eye open, the vitreous humor pushed against the retina so that it lay #at on the back of the eye. It may be all pushed together in a wad now.

The retina is made of cells that can detect light. The eye’s lens focuses the light that comes into the eye to make an image, a picture made of light. That image lands on the

retina. The cells of the retina react to the light that falls on them and send messages to the brain.

11. Use your "nger to push the retina around. The retina is attached to the back of the eye at just one spot. Can you "nd that spot? That’s the place where nerves from all the cells in the retina come together. All these nerves go out the back of the eye, forming the optic nerve, the bundle of nerves that carries messages from the eye to the brain. The brain uses information from the retina to make a mental picture of the world.

The spot where the retina is attached to the back of the eye is called the blind spot. Because there are no light-sensitive cells at that spot, you can’t see any light that lands in that place on the retina.

12. Under the retina, the back of the eye is covered with shiny, blue-green stu!. This is the tapetum. It re#ects light from the back of the eye.

Have you ever seen a cat’s eyes shining in the headlights of a car? Cats, like cows, have a tapetum (but people don’t). A

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cat’s eye seems to glow because the cat’s tapetum is re#ect-ing light. If you shine a light at a cow at night, the cow’s eyes will shine with a blue-green light because the light re#ects from the tapetum.

13. Look at the other side of the back of the eye. Can you "nd the optic nerve? To see the separate "bers that make up the optic nerve, pinch the nerve with a pair of scissors or your "ngers. If you squeeze the optic nerve, you may get some white goop. That’s myelin, the fatty layer that surrounds each "ber of the nerve.

When you’re done dissecting the cow’s eye, wrap all the pieces of the eye in plastic and throw them away. If you used a razor blade, dispose of it properly. A razor blade is only good for one or two dissections.

Aqueous humorA clear #uid that helps the cornea keep its rounded shape.

Blind spotThe place where all nerves from the retina join to form the optic nerve. Each eye has a blind spot where there are no light-sensitive cells.

ConesOne kind of light-sensitive cell in the retina. Cones give you color vision in bright light.

CorneaA tough, clear covering over the iris and the pupil that helps protect the eye. The cornea begins bending light to make an image; the lens "nishes the job.

IrisA muscle that controls how much light enters the eye. It is suspended between the cornea and the lens. A cow’s iris is brown. Human irises come in many colors, including brown, blue, green, and gray.

LensA clear, #exible structure that makes an image on the eye’s retina. The lens is #exible so that it can change shape, focusing on objects that are close up and objects that are far away.

MyelinThe fatty layer that surrounds each nerve "ber.

Optic nerveThe bundle of nerve "bers that carry information from the retina to the brain.

PupilThe dark circle in the center of your iris. It’s a hole that lets light into the inner eye. Your pupil is round. A cow’s pupil is oval.

RetinaThe layer of light-sensitive cells at the back of the eye. The retina detects images focused by the cornea and the lens. The retina is connected to the brain by the optic nerve.

RodsOne kind of light-sensitive cell in the retina. Rods respond in dim light.

ScleraThe thick, tough, white outer covering of the eyeball.

TapetumThe colorful, shiny material located behind the retina. Found in animals with good night vision, the tapetum re#ects light back through the retina.

Vitreous humorThe thick, clear jelly that helps give the eyeball its shape.



Have you ever done eye push-ups? What about eye-writing on a distant wall? Like all muscles, eye muscles get stronger if you exercise them. And while an “eye "tness” routine probably won’t let you throw away your glasses, it can prevent eyestrain, relieve eye tension, and help keep your eyes healthy and strong.

The exercises below are simple and don’t take much time. But just as you wouldn’t try to strengthen your abdominal muscles by doing a bunch of sit-ups once every two weeks, it’s better to do these exercises a little each day than to do them for long periods of time. And if you can do them throughout the day—maybe just 30 seconds of eye movement every couple of hours—it’ll be even better. When you’re waiting for your computer to boot up or riding the bus, try "tting in a few of these exercises. Just make sure you tell the people around you what you’re doing, or they might think you’re going crazy!

Eye Push-Ups1. Hold your "nger 10 inches away from your face. Slowly bring it towards your nose, stopping before it appears to double. Repeat 5 to 10 times.

2. Hold up your "nger in front of your face. Focus on your "nger and count to 3. Now focus on a point far away from you, counting to 3. Repeat a few times.

These exercises help strengthen your ability to change focus.

Eye WritingLook at a wall and pretend that you’re writing on it with your eyes. Don’t move your head—only move your eyes. Start by writing your name, then try composing poetry or writing out the lyrics to your favorite song. The bigger the letters, the more you strengthen and stretch the muscle around the eye.

‘Round the ClockImagine that you’re standing (or sitting) in front of a really big clock. Look at the middle of the clock. Then, without moving your head, look at any hour mark. Look back at the center. Then look at another hour mark, and come back to the center. Repeat for every “hour.” You can also do this exercise with your eyes closed. Either way will give the muscles around your eye a workout and help prevent eyestrain.

A Short Trip to the Twilight ZoneSitting comfortably, rub your hands together until they feel warm. Close your eyes and cover them lightly with your cupped palms. Leave no gaps between your "ngers and make sure no light gets through. Relax, take deep breaths, and focus on the darkness. After your eyes see nothing but blackness, remove your palms from your eyes. Repeat for a few minutes. This exercise is great for relaxing your eyes and relieving eyestrain.



• A room (like a bathroom) where you can turn on and o! the lights while looking in a mirror

1. Look at your eye in the mirror, paying special attention to the size of the pupil—the dark opening in the middle of the colored part of your eye.

2. Turn o! the lights and slowly count to ten.

3. Looking in the mirror, turn the lights back on, and watch what happens to the pupils of your eyes.

The pupil is actually a hole in the middle of your eye that lets in light. The more open it is, the more light can get in.

When you look at your pupil in bright light, it will be fairly small. After 10 seconds in a darkened room, the pupil will begin to enlarge, expanding to allow in as much light as possible. When the light goes on again, the pupil quickly contracts, shrinking from large back to small to reduce the light entering the eye.

Pupils respond quickly to any change in lighting. In just one-"fth of a second, they can expand from their smallest size to their largest. At its smallest, the pupil is just over one-thousandth of a square inch. At its largest, the pupil is up to "fty times that size.



Like "ngerprints, di!erent patterns in your eyes can be used to identify you. The patterns of the blood vessels in your retina, for example, are unique, as are the patterns on your iris (the colored part of your eye). Whether your iris is blue, brown, gray, green, or hazel, it’s "lled with streaks and spots of various colors. Even identical twins, who sometimes share identical "ngerprints, have individually unique irises.

People have been using "ngerprints for identi"cation for a long time, but eye recognition technology didn’t begin until 1935, when scientists proposed that the pattern of blood vessels in the retina might be unique to each individual.

For a while, researchers developed technology that could identify people by looking inside their eyes, at their retinas. But because it’s so much simpler to look at a person’s eyes from the outside, iris scanning proved an even better system of identi"cation.

In 1992, an ophthalmologist, Dr. John Daugman, created a mathematical formula for measuring the characteristics of irises. Because the iris remains unchanged throughout life, it’s considered more reliable than "ngerprints, which

can be damaged or altered by injury or wear.

The iris is made up of connective tissue and smooth muscle "bers that open or close to control how much light enters the pupil, much in the same way that a shutter controls the light entering a camera lens. The iris dilator muscle, which radiates like spokes on a wheel, creates the unique pattern of each iris.

Of course, you can’t use ink and paper to make an iris-print the way you can a "ngerprint. Instead, an iris identi"cation system typically uses a high-quality digital camera and infrared light source to capture an image of a person’s eye. Then, a circular grid is overlaid on the image so that dark and light patterns can be reduced to points of identi"cation. These points create an “eyeprint.” Although this may sound like a lot of work, a computerized iris identi"cation system can complete the process in two seconds.

Iris identi"cation systems are already being used in high-security areas, and some banks are scanning irises at ATM machines instead of using cards and passwords. Experts predict that, someday, your home computer may be able to look you in the eye and know exactly who you are.



• A penlight

1. Close your eyes.

2. Gently press the lighted penlight against the outer edge of your upper eyelid. Keep the penlight gently pressed against your closed eye, and move your eyes back and forth.

3. Move the penlight around a bit if you need to. It may take a few moments, but when you "nd the right place, you’ll see a pattern of dark, wavy lines. These are the shadows of blood vessels inside your eye.

The lines you see are the shadows of blood vessels in your eye. These shadows are always in your "eld of vision, but normally you don’t notice them.

Almost any stimulus that doesn’t change will eventually be ignored: You don’t pay attention to the feel of the shoes on your feet or the tip of your nose in your "eld of vision.

In this case, the nerve receptor cells on your retina (the lining at the back of your eye) receive a constant stimulus from these blood vessels, begin to fatigue, and stop reacting. When you move your eyes under the penlight, you create moving shadows of these blood vessels on your retina. Nerve receptors respond to this new stimulus, and you see the shadows of the blood vessels in your eye.



• A 3-x-5-inch (8 x 13 cm) card or other sti! paper• Pen, pencil, or marker

1. Make a dot on the left side of the card and an X on the right side, as shown below (or use this page).

2. Hold the card at eye level and arm’s length away, making sure the X is on the right side.

3. Close your right eye and look directly at the X with your left eye. Notice that you can also see the dot.

4. Keep focusing on the X as you slowly bring the card toward your face. The dot will disappear, and then reappear, as you bring the card toward your face.

5. Now close your left eye and look directly at the dot with your right eye. This time the X will disappear and reappear as you bring the card slowly towards your face.

At the back of each eye is a light-sensitive lining called the retina. Cells in the retina receive incoming light and then send signals to your brain, allowing you to see.

However, there’s one small spot on your retina that has no light-sensing cells. That spot—called the blind spot—is where your optic nerve connects your eye to your brain.

When you move the card to the exact spot in your vision where the dot or X seem to disappear, you’ve found the blind spot.

eye innards

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