Regeneration- to make something new that was old or used.
The photoreceptors are located at the back of the eye in a thin layer
called the retina. If you think of the eye as a camera, the retina would
be the film. The retina also contains the nerves that tell the brain
what the photoreceptors are "seeing." Take a close look at the
photoreceptors in the figure below. They seem to be set up backwards!
Light is absorbed by the outer segments of the photoreceptors. These
are stuck in the back of the retina. They are embedded in a layer of
cells called the retinal pigmented epithelium (RPE). Why? The outer segments
are made of stacks of discs containing the molecule rhodopsin. This is the
molecule that actually absorbs light. The discs are discarded all the time.
This allows the rhodopsin to be regenerated and reused. The regeneration
process is the job of the RPE. To regenerate the rhodopsin the RPE needs
vitamin A. Carrots are one food high in vitamin A and why you have
heard "eat your carrots, they are good for your eyes."
We have three types of cones. If you look at the graph below, you can see
each cone is able to detect a range of colors, but at lower
sensitivity. Even though
each cone is most sensitive to a specific color of light they also can
detect other colors. In addition, they overlap each other. The overlap allows use to see many different colors.
Since
the three types of cones are commonly labeled by the color they are
most sensitive (blue, green and red) you might think other colors are not possible. In
fact, it is the overlap of the cones and how the brain integrates the
signals sent from them that allows us to see millions of
colors. For example, the color yellow results from green and
red cones being stimulated while the blue cones have very little
stimulation.
* The
wavelength of light determines the color we see. The color spectrum
ranges from 380nm to 750nm.
So how do we see the color white?
Our eyes are detectors. When the cones are stimulated by light
they send signals to the brain. The brain is the actual interpreter of
color. When all the cones are stimulated equally the
brain perceives the color as white. We also perceive the color white
when our rods are stimulated. Unlike cones, rods are able to detect light at
a much lower level. This is why we see only black and white in dimly lighted
rooms or while out viewing a star filled night sky.
The photoreceptors are located at the back of the eye in a thin layer
called the retina. If you think of the eye as a camera, the retina would
be the film. The retina also contains the nerves that tell the brain
what the photoreceptors are "seeing." Take a close look at the
photoreceptors in the figure below. They seem to be set up backwards!
Light is absorbed by the outer segments of the photoreceptors. These
are stuck in the back of the retina. They are embedded in a layer of
cells called the retinal pigmented epithelium (RPE). Why? The outer segments
are made of stacks of discs containing the molecule rhodopsin. This is the
molecule that actually absorbs light. The discs are discarded all the time.
This allows the rhodopsin to be regenerated and reused. The regeneration
process is the job of the RPE. To regenerate the rhodopsin the RPE needs 
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