????????????????????????????????
SCIENCE MADE SIMPLE
Some people can see the world around them very well. To others, everything looks blurry without their eyeglasses. Have you wondered:
Why do some people need glasses?
LIGHT BENDS WHEN IT MOVES FROM
ONE MATERIAL INTO ANOTHER. THIS
IS WHY A STRAW LOOKS BENT OR
BROKEN WHEN YOU PUT IT IN A
GLASS OF WATER.
Your Eyes
Sight is one of the five senses. You use your senses to learn about the world around you.
Your eyes collect and focus light. This light brings you information about your surroundings. When light reaches the back of your eyes, it is turned into nerve signals. The signals are sent to your brain, and you can see.
How much does the light bend? That depends on what it travels through. The speed of light is different in different materials. Light travels faster in air than in water or glass. The greater the change in speed between the materials, the more the light bends.
AIR
Light
You need light to see, but what is light? Light is a kind of energy. It moves from one place to another in a straight line.
Some things produce light. You can see the sun or a light bulb when the light they give off reaches your eyes.
But most things do not produce light. How can you see them? You see reflected light, which has bounced off their surface.
Refraction - Bending Light
Some materials, like water or glass, are transparent. You can see through them because they transmit light (let it pass through). When light moves from one material into another it changes direction, or bends. This is called refraction.
WATER DIAMOND
LIGHT BENDS MORE IN DIAMOND THAN IN
WATER BECAUSE IT TRAVELS MORE SLOWLY.
FUN FACTS
What is 20/20 vision?
Many people believe that “20/20” vision means
“perfect” vision. It does not. If you have 20/20 vision, you can see clearly at 20 feet what a “normal” eye can see clearly at 20 feet. If a normal eye can see something at 50 feet, but you can only see it clearly at 20 feet, you have 20/50 vision.
Some people have better than normal distance vision. For example, 20/15 vision means that you can clearly see something from 20 feet away, while a person with normal vision would have to be closer
(15 feet away) to see it.
© Copyright 2004, 2007 by Science Made Simple, Inc.
PO Box 503, Voorhees, NJ 08043 sciencemadesimple.com
science@sciencemadesimple.com

SCIENCE MADE SIMPLE Eyeglasses
How much light bends also depends on the angle it makes with the surface. If the light rays are at a 90 degree angle (perpendicular) to the surface, they do not bend. They just continue travelling in the same direction. When the light rays are at an angle, they bend. The greater the angle, the more the light bends.
Then, they bend again as they leave. On the other side, the light rays have been bent so that they are focused (or come together) at one point.
This is called the focal point.
FOCAL
POINT
PERPENDICULAR
LIGHT IS NOT BENT.
AT GREATER ANGLES
LIGHT BENDS MORE
Focusing Light
This bending of light can be very useful. Let’s look at what happens when light reaches a curved surface, like the lens of a magnifying glass.
In the drawing below, the rays of light are all going straight towards the lens.
In the real world, light usually comes from all different directions. The lens bends and focuses the light to make an image (or picture) of objects on the other side.
OBJECT IMAGE
OBJECT
LIGHT
RAYS
IMAGE
As they enter the glass, they slow down. Light reaching the center of the lens is perpendicular to the surface. It goes straight through.
All the other light rays reach the surface at an angle. They bend as they enter the lens.
Inside your eye
An adult’s eye is about 2.5 cm (1 inch) across.
It is roughly spherical, with a small bulge in front. It weighs about 7 grams (about 1/4 ounce). The eyeball is covered by a hard, white layer called the sclera. Muscles attached to the sclera are used to move the eye.
The front of the eye is attached to the inside of the eyelid by a white membrane called the conjunctiva. (A membrane is a thin, soft bendable layer.) This is what we see as the
“white” of your eye.
Light enters the front of the eye through the cornea. It is a clear, tough covering, curved like the lens of a magnifying glass. Light passing through the cornea slows down, and bends towards the center. The cornea does most of the focusing work for the eye.
page 2

SCIENCE MADE SIMPLE Eyeglasses
Behind the cornea is the iris. It is the donutshaped colored part of your eye. The dark spot in the center is an opening called the pupil. The pupil lets light through to the inside. The muscles of the iris control the size of the opening. In bright light, the pupil contracts (or gets smaller). It expands (gets bigger) in the dark, to let in as much light as possible.
Iris
Retina
Sclera
Pupil
Cornea
Vitreous
Humor
Aqueous
Humor
Optic
Nerve
Why do people need eyeglasses?
Sometimes eyes do not work perfectly. People wear glasses to correct many kinds of problems.
Most eyeballs are roughly spherical. Some people have eyeballs that are too long. They are nearsighted. Light from far away is focused before it reaches the retina. Distant objects look blurry. Eyeglasses can spread out (diverge) the light rays, so that they focus farther back in the eye on the retina.
NEARSIGHTED -
IMAGE IS FOCUSED
IN FRONT OF RETINA
LENS DIVERGES
LIGHT - IMAGE
IS FOCUSED ON
THE RETINA
The space between the cornea and iris is filled with a clear liquid called the aqueous humor.
Behind the iris is the lens. The lens is clear, and about the size of a small bean. By changing shape, it controls fine focusing. Light rays must be bent sharply to clearly see nearby objects. The muscles holding the lens contract (tighten and get smaller). The lens becomes thicker and more curved. Light bends more, and is focused on the back of the eye. Light from distant objects
(farther than about 6 meters or 20 feet) does not have to bend as much. The muscles holding the lens relax. The lens becomes thinner and flatter, and light is focused on the back of the eye.
Behind the lens is the vitreous humor. It is a clear, jellylike material, that makes up about twothirds of the eye’s volume.
The focused light finally reaches the retina at the back of the eye. The retina holds millions of light-detecting cells, called rods and cones. The cones are used in bright light and color vision.
The rods are used in dim light, and only sense in black and white. When light hits the rods and cones, they send a signal to the brain. Then you can see the world around you.
NORMAL EYE - IMAGE IS FOCUSED ON THE RETINA
Some people are farsighted. Their eyeballs are too short. Light from nearby is focused behind the retina. Close objects look blurry.
Eyeglasses can bend the light rays together
(converge the light), so that they focus closer to the front, on the retina.
FARSIGHTED -
IMAGE IS FOCUSED
BEHIND THE RETINA
LENS CONVERGES
LIGHT - IMAGE
IS FOCUSED ON
THE RETINA
Astigmatism is a common problem usually caused by an uneven cornea. Images reaching the retina are distorted. Glasses can correct for this by bending light rays different amounts, depending on where the light hits the lens.
Farsightedness is a common problem when people get older. The lens in your eye continues to grow and becomes stiffer. It gets harder to focus on nearby objects. Reading glasses magnify print, making it easier to see.
Some people have eyes that can not focus well either near or far. They can use “bifocals” to correct both problems at once. They can look through the bottom part of the lenses to read, and the top part to see distant objects.
page 3

SCIENCE MADE SIMPLE Eyeglasses
The Photoreceptive Cells
Blue sky. Green grass. Yellow daffodils. We see the world in vivid color. At night, these colors seem to vanish, leaving only shades of grey. Where do the colors go?
The human retina contains about 150 million photoreceptive, or light-detecting, cells. There are two types: rods and cones. They get their names from the difference in their shapes. The
7 million cones are used for daylight vision.
They can detect different colors, and send clear, sharp images to the brain.
There are 100 million to 150 million rods. They are used in dim light and night vision. Rods sense only in black and white, and cannot detect color. They are much more sensitive to low light levels than the cones, but the signals they send to the brain are not as sharp. This is why you only see in shades of grey at night, and the images may seem fuzzy.
GANGLION
CELLS called the fovea. The fovea is about 1 mm in diameter, or about the size of a pinhead. It contains only cones (between 10,000 and 30,000) and is the most sensitive part of the eye. Your central vision is sharp and clear. But the fovea has a limited field of view, about 4 square inches at 8 feet (about 26 square centimeters at 2.4
meters). As you focus on an object, your eyeballs continually move to keep the image lined up on the fovea.
Your retina covers about 65% of the inside of your eyeball. Beyond the macula, the rest of the retina has mostly rods. Your peripheral vision is less sharp than your central vision, but it is still very important. It responds more to movement and changes than to a fixed scene.
You stay aware of your surroundings without being overwhelmed by needless information.
It is also necessary for night vision. Have you ever tried to look directly at a star, only to have it seem to disappear? Your central vision is useless at night. The cones in your fovea cannot detect the dim starlight. You can see the star when you look a little to the side and the light falls on the rods around the fovea.
The Macula and Fovea
The rods and cones are not evenly spaced throughout the retina. Most of the cones are located in a small central spot, called the macula.
In the center of the macula is a small indentation
CONE RODS
OPTIC NERVE FIBERS
LIGHT
CONE
BIPOLAR
CELLS
Light Sensitive Pigments
How do rods and cones detect light? Each cell holds several million molecules of light sensitive pigment.
The pigment in the rods is called rhodopsin, or visual purple. It has two parts. The first is a large protein molecule, called an opsin. The second is a smaller organic molecule, called retinal. (Retinal is closely related to Vitamin A.)
When the pigment absorbs light, the retinal changes shape. It straightens out and breaks apart from the protein. The rod produces a short electrical pulse or signal. This signal is passed along through other cells to the optic nerve, and then to the brain.
The pigments in the cones are called iodopsins. Like rhodopsin, the iodopsins contain a protein and retinal, and work the same way. There are three types. Each absorbs a range page 4

SCIENCE MADE SIMPLE Eyeglasses of wavelengths, but is most sensitive to a different color, either blue, green or red, depending on the structure of its protein. These three pigments let us distinguish up to 200 different colors.
Connections to the Optic Nerve
The retina is about 1/2 mm (1/50 inch) thick.
The rods and cones are actually located at the back of the retina. The light must pass through a maze of nerves and other cells before reaching them.
In front of the light-detecting cells is a layer of bipolar cells. Each bipolar cell may be connected to many rods and cones. The next layer contains ganglion cells. Each is connected to several bipolar cells. Concentrating the signals from many cells into one cell is called convergence.
The optic nerve fibers are attached to the ganglion cells. They join together in the optic nerve, pass through the retina and go to the brain. Although there are about 150 million rods and cones, there are only about 1 million optic nerve fibers.
Convergence is an important part of the difference between rod and cone vision. The cones in the fovea have almost a one-to-one connection to the bipolar cells. This produces very sharp, clear images from your central vision.
The rod system is much more sensitive to dim light for two reasons. First, the individual rod cells are more sensitive to light than cone cells.
Second, there is more convergence in the signals from the rods. A weak signal from many rods attached to a single bipolar cell can be as effective as a stronger signal from fewer rods. But because of the overlapping signals, images from the rods are not as clear and sharp.
The Blind Spot
There is a small spot on each retina that has no rods or cones. This is the place where the optic nerve passes out the back of the eyeball on its way to the brain. It is called the blind spot.
You normally do not notice your blind spots because the view from both eyes overlaps. The brain fills in the image for each blind spot with information from the other eye.
1) What do you see in this picture?
2) Which line is longer?
3) Which circle is larger?
4) Compare the height of the hat to the width of the brim.
5) Which ring is on top?
page 5
6) Are these vertical lines curved, or straight and parallel?
(SEE PAGE 8 FOR THE ANSWERS)

SCIENCE MADE SIMPLE Eyeglasses
Most people can see very well.
Other people have to wear eyeglasses.
Have you ever wondered why?
Your eyes are like a camera. They collect and focus light to make a picture of things around you.
What is light? Light is a kind of energy. It moves in straight lines.
Some things, like glass or water, are clear. They let light go right through them. But the light does not go straight through. It bends, or changes direction. This is why a straw looks bent or broken when you put it in a glass of water.
Your eyes also bend light. In the front of your eye is a clear, tough covering. It is called the cornea. Light going through the cornea gets bent and focused.
The colored part of your eye is the iris. The dark spot in the middle is an opening called the pupil. It controls how much light gets inside your eye.
The pupil opens wide in the dark. It gets smaller in bright light.
Behind the pupil is the lens. It is clear, and about the size of a small bean.
It changes shape, to bend and focus light. When you look at things nearby, the lens gets rounder. To see things far away, the lens gets flatter.
The focused light makes a picture on the back of your eye. There, the retina records the picture and sends it to your brain. This is how you see.
So, why do people need glasses?
Because their eyes are not perfect.
Light does not focus on the retina like it should. Things look fuzzy.
Some people are “near-sighted.”
Their eyeballs are too long. Others are
“farsighted.” Their eyeballs are too short. Many people have uneven corneas. Eyeglasses can correct these problems. Glasses bend the light, so it is focused in the right place. Then people can see the world clearly.
NORMAL EYE -
PICTURE IS
FOCUSED ON
THE RETINA
Iris
Retina
NEARSIGHTED -
PICTURE IS
FOCUSED IN
FRONT OF RETINA
Pupil
Cornea to the
Brain
GLASSES SPREAD
OUT LIGHT, SO THE
PICTURE IS FOCUSED
ON THE RETINA page 6

SCIENCE MADE SIMPLE Eyeglasses
SAFETY NOTE: Read all instructions completely before starting. Observe all safety precautions.
PROJECT 1 - Find Your Blind Spot
What you need: the drawing of two circles shown below
What you do:
(1) Hold this page at a comfortable arms length away.
(2) Cover your left eye and focus your right eye on the light grey circle.
(3) Move the page closer and farther away until the dark grey circle disappears.
(4) Now cover your right eye, focus on the dark grey circle and move the page until the light grey circle disappears.
What happened: The image of the circle seems to vanish when it falls on the blind spot, the place where the optic nerve passes through the retina. You normally do not notice your blind spot.
When both eyes are open, their fields of view overlap, and they fill in the missing information for each other.
PROJECT 2- See How Your Pupils Expand and Contract
What you need: a mirror, a bright light with a switch
What you do:
(1) In a brightly lighted room (a bathroom works well) look closely at your eyes in the mirror.
Your pupils will be very small.
(2) Turn off the light (leave the door open to get some light in.) Let your eyes adjust to the dim light, then look at your eyes in the mirror. What happened to your pupils?
(3) While still looking in the mirror, turn the light back on. Now what happens to your pupils?
What happened: The iris contains pigments to block light rays from entering the eye. All light must pass through the pupil. When too much light reaches the back of the eye, nerve signals are sent to the muscles of the iris, making the pupil contract. In dim light, the pupil opens wide to let more light in. Your pupils can go from about 8 mm (0.33 inch) wide, down to about 3.5 mm (0.13
inch.) If the light is still too bright, you will automatically squint or close your eyes.
page 7

SCIENCE MADE SIMPLE
PROJECT 3- Check for astigmatism
What you need: the drawing on the right
What to do:
(1) Hold the page a comfortable reading distance away.
(2) Cover one eye and focus on the drawing to the right. Note which lines are sharp and clear, and which lines are blurry.
(3) Repeat steps (1) & (2) with the other eye.
(4) If you wear glasses, take them off and repeat steps (1) through (3).
What happened: Most people have some degree of astigmatism. Astigmatism is usually caused by an imperfect cornea. Some areas of the cornea bend light more than others. This makes parts of the image look out of focus. Do you have astigmatism, or did the whole drawing appear sharp and clear? If you wear glasses, do they correct the astigmatism?
Eyeglasses
Vision is a very complicated process. It starts with light reaching the eye. The eye sends electrical signals to the brain. The brain then processes the signals, and creates images of the world around you. To make sense of all the information it gets, your brain depends on past experience. It also uses many visual clues, like relative shapes and sizes, contrast, colors, light and shadows.
Sometimes, the information is confusing, and can be interpreted in more than one way. This is an optical illusion.
1) Do you see a vase, or two faces in profile? The image may actually flip from one to the other as you stare at it. This illusion is caused by confusing visual signals. Your brain does not know whether to see the white area as the background or the foreground. This is called object reversibility.
2) Both lines are the same length. The arrows extending outward make the bottom line look longer.
3) Both circles are actually the same size. When you look at the picture, your brain compares the circles to the lines in the background. The one touching the lines seems larger.
4) The height of the hat and width of the brim are equal. The brain “sees” vertical lines as longer than horizontal lines. Broken or intercepted lines seem shorter than unbroken lines. And the larger area or bulk of the top section of the hat compared to the brim adds to this illusion.
5) This is an example of reversible perspective, which happens when the brain gets confusing but equally acceptable clues. Either the right or the left circle can appear to be on top, and the brain switches from one view to the other.
6) The lines are straight and parallel. The two lines on the left appear to spread apart in the middle, because the short diverging lines lead the eyes outward. The two lines on the right appear to bend together in the middle because the short lines lead the eyes inward.
page 8