Station 3: Rods & Cones As you look at the world around you, electromagnetic waves of light enter the eye
through the cornea. The cornea is a clear dome at the front of the eye that bends and
converges light towards the pupil, which is the circular opening in the center of the
colored iris. In low light conditions the pupil will dilate, or get bigger to let more light
in. In intense light conditions the pupil will constrict, or get smaller to limit the amount
of light getting through. Immediately after the pupil, light waves will be bent and
converged once again as it passes through the crystalline lens, through the vitreous
humor, or clear gel that makes up about 80% of the eye’s volume, and towards the retina
located on the back of the eye.
Within the layers of the retina are special sensory receptors, called photoreceptors that
respond to the electromagnetic waves of visible light. There are two types of
photoreceptors: rods and cones, that they are responsible for converting light waves into
electrical signals that can be sent to the brain via the optic nerve.
There are about 120 million rods on the retina that are very sensitive to light. However,
they are not sensitive to color. Rods are responsible for low light vision. Cones are far
fewer in number with only 6 to 7 million and are less sensitive to light. However, cones
are responsible for all high-resolution vision and provide the eye’s color sensitivity.
Cones can be divided into red cones, green cones, and blue cones based on light
wavelength sensitivity.
Color Blindness Color blindness is the inability or decreased ability to color or perceive color differences.
Color blindness can sometimes be caused by physical or chemical damage to the eye, the
optic nerve, or certain parts of the brain. Color blindness can also be inherited, passed on
from generation to generation. There are three main types of color blindness: total color
blindness, red-green color blindness, and blue-yellow color blindness.
Total color blindness, or Achromatopsia, is defined as the complete inability to see color
and can only perceive variations in brightness. This condition is extremely rare. This
image below depicts the comparison between the field of view of an individual with
normal vision and an individual with achromatopsia.
Red-green color blindness is the most common form of color blindness in which
individuals have trouble discriminating between the small difference in red, orange,
yellow and green end of the spectrum of visible light.
o Protanomaly (red-weakness), deuteranomaly (green-weakness), protonapia, and
deuternopia are all forms of red-green color blindness. The image below depicts the
comparison between the field of view of an individual with normal vision and
individuals with the different forms of red-green color blindness.
The image below depicts the comparison between an individual with normal
vision and an individual with protanomaly, or “red-weakness”. Any redness seen
in a color by a normal vision individual is seen more weakly with someone with
protanomaly. The redness component in violet or purple for example will be so
weakened that an individual with this condition will think it is just another shade
of blue
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Blue-yellow color blindness​
is far less common and occurs when individuals
often confuse blue with green wavelengths of light and yellow with violet.
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Trianopia and tritanomaly are forms of blue-green colorblindness in which
cones that are sensitive to the shorter wavelengths of light are either missing or do
not function correctly. The image below depicts the comparison between the field
of view of an individual with normal vision and individuals with the different
forms of blue-yellow color blindness.
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The image below depicts the comparison between an individual with normal
vision and an individual with trianopia, a form of blue-green color blindness.
The Ishihara color test, which consists of a series of pictures of colored spots, is the
most commonly used test to diagnose red-green color blindness. Since
blue-yellow colorblindness is so rare, there is no commonly available test for it. In
this activity you and your partners will take the Ishihara color test.
Station 4: Finding Your Blind Spot Every time you open your eyes light enters through the cornea, passes through the
pupil and lens, to stimulate the rods and cones (photoreceptors) located within the retina
at the back of the eye. When stimulated the rods and cones send electrical signals to the
brain via the optic nerve. This is how you are able to see all the things in front of you.
There is, however, a section on the back of the eye where the optic nerve connects to the
retina in which no photoreceptors are present. This is called the b
​lind spot​
. Since there
are no rods or cones present here, we’re not able to interpret any light shone in this
section, hence the name “blind” spot.
Technically, the blind spot should appear as black areas in our field of vision (think of
what you see when there is no light present). Obviously, this is not the case. You are
able to read all the words on this sheet of paper and see all the objects in your
surrounding without any noticeable black spots in your field of vision. So why is this?
The main reason why you do not notice your blind spot is because you have two eyes.
Your left eye and right eye actually send two slightly different images to your brain.
Try closing one eye and alternate to the other while looking at this sheet of paper. You
will notice that the words and pictures are not quite in the exact same place when you
switch from eye to eye. The individual images sent down the optic nerve from your left
and right eye are merged together by your brain to produce a single image.
Fortunately, the blind spot on your left eye is not in the same location as your right eye.
So when the images from each eye overlap in your brain, the blind spots are covered by
the opposite eye.
But what about when you have one eye closed? Why is the blind spot not noticeable
then? This is because your brain has the ability to assess the background of any image
sent from each eye and “fill in” any areas where information is missing. Try the
following exercises to find the blind spot for each eye and discover the brain’s amazing
ability to compensate for them.