Your retina is packed with light-sensing cells. But you may not realize that these light detectors come in two flavors:
• Rods. These are the most numerous light-sensing cells in your eyes. They’re extremely efficient at collecting light and continue working even in near-darkness. For this reason, your rods power your night vision. However, rods don’t distinguish between different colors.
• Cones. These cells are less sensitive than rods (meaning they need more light to do their work), but they’re able to perceive fine detail and color. Your eyes have three types of cones, which give you the ability to perceive three primary colors (red, green, and blue), provided there’s enough light for your cones to function.
Rods are spread throughout the sides of your eye. Cones are packed into a small region in the middle of your eye.
This combination of rods and cones gives your eye the best of both worlds—highly sensitive black-and-white vision you can use to creep around at night, and sharp color vision that catches more detail during the day. Although your brain switches effortlessly between the two, it takes 30 minutes of darkness to develop your best night vision. That’s because night vision requires chemical changes in your eye that make your rods more sensitive.
A short blast of ordinary light usually resets your vision from dark- to lightadjusted. However, dim red light doesn’t trigger this change because your dark-adjusted rods are naturally tuned to the blue end of the light spectrum, and they have a hard time picking up the color red. That’s why astronomical observatories and submarines use red light to illuminate dials and switches—it provides enough light for operators to see their instruments in a darkened environment, but it doesn’t disrupt their night vision.
Incidentally, most color-blind people have the same three types of cones as everyone else, but their cones don’t function normally. These people may have trouble discriminating between certain shades of color, but the effect is usually minor. Often, it goes unnoticed until they try a color-vision test (like the spot-the-number test at http://en.wikipedia.org/wiki/Ishihara_color_test). However, people with rarer, more serious forms of color-blindness may have only two functioning types of cones. They may be unable to distinguish purple from blue or red from yellow, except by the slight changes in brightness of the hues, not by the colors themselves.
Source of Information : Oreilly - Your Body Missing Manual