The Retina

The retina is the back part of the eye that contains the cells thatrespond to light. These specialized cells are called photoreceptors. There are 2 types of photoreceptors in theretina: rods and cones.

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The rods are most sensitive to light and dark changes, shape andmovement and contain only one type of light-sensitive pigment. Rods arenot good for color vision. In a dim room, however, we use mainly ourrods, but we are "color blind." Rods are more numerous than cones in theperiphery of the retina. Next time you want to see a dim star at night,try to look at it with your peripheral vision and use your ROD VISION tosee the dim star. There are about 120 million rods in the humanretina.

The cones are not as sensitive to light as the rods. However, cones aremost sensitive to one of three different colors (green, red or blue).Signals from the cones are sent to the brain which then translates thesemessages into the perception of color. Cones, however, work only inbright light. That"s why you cannot see color very well in dark places. So, the cones are used for color vision and are better suited fordetecting fine details. There are about 6 million cones in the humanretina.
Some people cannot tell some colors from others - thesepeople are "color blind." Someone who is color blind does not have aparticular type of cone in the retina or one type of cone may be weak. In the general population, about 8% of all males are color blind and about0.5% of all females are color blind.

The fovea, shown here on the left, is thecentral region of the retina that provides for the most clear vision. Inthe fovea, there are NO rods...only cones. The cones are also packedcloser together here in the fovea than in the rest of the retina. Also,blood vessels and nerve fibers go around the fovea so light has a directpath to the photoreceptors.

Here is an easy way to demonstrate the sensitivity of your foveal vision. Stare at the "g" in the word "light" in middle of the following sentence:

"Your vision is best when light falls on the fovea."

The "g" in "light" will be clear, but words and letters on either sideof the "g" will not be clear.

One part of the retina does NOT contain any photoreceptors. This isour "blind spot." Therefore any image that falls on thisregion will NOT be seen. It is in this region that the optic nerves cometogether and exit the eye on their way to the brain.

To find your blind spot, look at the image below or draw it ona piece of paper:


Close your left eye.

Hold the image (or place your head from the computer monitor) about 20inches away. With your right eye, look at the dot. Slowly bring theimage (or move your head) closer while looking at the dot. At a certaindistance, the + will disappear from sight...this is when the + falls onthe blind spot of your retina. Reverse the process. Close your right eyeand look at the + with your left eye. Move the image slowly closer to youand the dot should disappear.

Here is another image that will help you find your blind spot.


Forthis image, close your right eye. With your left eye, look at the redcircle. Slowly move your head closer to the image. At a certaindistance, the blue line will not look broken!

Did you know?movie Why can"t you see very well when youfirst go into a darkened room like a movie theater? When you first enterthe movie theater, the cones in your retina are working and the rods arenot yet activated. Cones need a lot of light to work properly; rods needless light to work, but they need about 7-10 minutes to take over for thecones. After 7-10 minutes in the dark, the rods do work, but you cannotsee colors very well because the rods do not provide any colorinformation. The cones, which do provide color information, need morelight, but do not work well in the dark. After the movie is over and youleave the theater, everything looks very bright and it is hard to see fora minute or two. This is because the rods become "saturated" and stopworking in these bright conditions. It takes a few minutes for the conesto begin to function again, and for normal vision to be restored.

Acomplete lesson plan on the eye and itsconnections - teacher and student guides available. Also, try some experiments to test your sense of sight and take a short, interactive quiz about the eyeand sight.

See more: Los Angeles To Las Vegas Distance Between La And Las Vegas From Los Angeles

Find out more about blind spots, vision, the retina and photoreceptors.

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