by Kim Cooper
and C.J. Kazilek
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Words to know before you read
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- Ishihara test- Named after its inventor, this test is
used to detect your ability to see colors.
- Photon- the smallest bit of light
- Photoreceptor- the special type of cell in your eye that picks up
photons and then signals the brain. They are located in the retina (a
layer at the back of the eye). There are two types, rods and cones.
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- Prism- A crystal object, such as cut glass, with at least
three similar sides.
- Trillion- 1,000,000,000,000
- Wavelength- a property of photons that determines
their energy (how "strong" they are).We see photons of different wavelengths
as different colors.
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Most images in this article are linked to more information.
| Why Roses are Red and Violets are Blue? Did you ever wonder why you see the colors you do or if other animals see the
same colors that you see? We see light that bounces off of things around us.
When the light enters our eyes, special cells tell our brains about the light.
These cells are called photoreceptors. Light is made of little bits called photons.
When the sun shines, trillions and trillions of these little bits of light
fall on the earth. The photons bounce off of almost everything and some of
them enter our eyes. Those bits that enter our eyes allow us to see.
So, where does the color come from?
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Listen to the
Ask-a-Biologist
Roses are Red and Violets are
Blue, but Why? Podcast
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Starting in the 1600s with Sir Isaac Newton,
scientists have believed that there are different kinds of photons.
Different types give rise to our sense of colors. The different
photons are said to have different wavelengths. Sunlight contains all
the different wavelengths of photons. The visible wavelength colors
can be seen when you look at a rainbow. Raindrops acting as natural
prisms produce the colors.
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| Where can you find
more information about rainbows? Just place
your mouse pointer at the end of this rainbow and click. |
How do our photoreceptors work?
 We have two main types of photoreceptors called rods and cones.
They are called rods and cones because of their shapes.
These cells are located in a layer at the back of the eye called the
retina. Rods are used to see in very dim light and only show the world to us
in black and white. This is why you see only black and white when you are
outside in the evening or in a dimly lit room. The other type of photoreceptors, the
cones, allow us to see colors. They are not as sensitive as the rods so they
only work in bright light. There are three types of cones, one for each
of the three main colors we see, red, green and blue.
(click on the eyes above to learn more)
Some people have a genetic defect that makes one or more of the cones fail.
This condition is known as color deficiency. You may have heard it called
color blindness. Color blindness is fairly common, affecting about nine percent of all
humans. It is much more common in men than in women. To test for color blindness a
special picture called an Ishihara test is used.
If you jump to our color test page you will be able to
test yourself and also
experience another interesting phenomenon of our color vision.
What about other animals? What kind of colors do they see? Most animals see
fewer colors than we do, but some see more! We know this by looking at how many
kinds of cone photoreceptors they have. Another good indication of what an animal can
see is by looking at their own colors. The colors of their prey are also an indication
of an animals ability to see color. We have made a table of some common animals
and what colors they see.
Here are some other places to read and learn about color vision.
On the Web:
There are hundreds of sites on the web, but their web addresses often change.
It is best to look for them using a search engine like
HOTBOT using the key
words color vision, color blindness, Ishihara test.
Books:
How Animals See, by Sandra Sinclair, Facts on File
Publications, New York, 1985
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