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SCIENCE MADE SIMPLE
WHY DO PEOPLE NEED EYEGLASSES?
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?
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.
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.
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.
DIAMOND
WATER
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.
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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.
PERPENDICULAR
LIGHT IS NOT BENT.
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
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.
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.
OBJECT
IMAGE
LIGHT
RAYS
LENS
OBJECT
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.
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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.
Retina
Iris
Sclera
Aqueous
Humor
Lens
Pupil
Cornea
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
Vitreous
Humor
Optic
Nerve
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
LENS DIVERGES
LIGHT - 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.
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LEARN MORE ABOUT:
RODS & CONES
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.
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
CONE
BIPOLAR
CELLS
GANGLION
CELLS
OPTIC NERVE FIBERS
LIGHT
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.
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
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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.
FUN FACTS
OPTICAL ILLUSIONS
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.
6) Are these vertical
lines curved, or
straight and
parallel?
5) Which ring is on top?
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(SEE PAGE 8 FOR THE ANSWERS)
SCIENCE MADE SIMPLE
Eyeglasses
I CAN READ
WHY DO PEOPLE NEED 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.
NORMAL EYE PICTURE IS
FOCUSED ON
THE RETINA
NEARSIGHTED PICTURE IS
FOCUSED IN
FRONT OF RETINA
Retina
Iris
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.
Cornea
Lens
Pupil
to the
Brain
GLASSES SPREAD
OUT LIGHT, SO THE
PICTURE IS FOCUSED
ON THE RETINA
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SCIENCE MADE SIMPLE
Eyeglasses
PROJECTS TO DO TOGETHER
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.
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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?
LEARN MORE ABOUT OPTICAL ILLUSIONS
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.
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