Why are some people color-blind?

We recognize color because we have special receptors behind the retina at the back of the eye. Light enters the lens, passes  through the whole of the retina, and strikes these receptors, which are divided into rods and cones.

Color perception depend mainly on the cones, which are of three kinds, each containing different color-sensitive pigments. One responds best to red (long wavelength) light, another to green (intermediate) and the rest to blue (short). These correspond  to the primary colors in an artist’s paintbox. All the colors we see – between 120 and 150 hues – result from the combined  stimulation of these cones, a mixing of the primary colors in the eye’s palette.

The lack of one kind of cone brings a failure to see a particular colour. Total color blindness, in which everything appears in shades of grey, is extremely rare. What we call colour blindness is usually an inability to distinguished from green. It can be caused by injury to the optic nerve and retina, but most people inherit it. It affects about 8 percent of white European males and less than 1 percent of females; it is less prevalent among Asians and even rarer among blacks. Women who are not themselves colour-blind may pass the defect to some of their children.

In a severe case, somebody with green blindness has difficulty distinguishing oranges, greens, brown and pale reds. Those with a red deficiency never see brilliant reds; all reds are duller.

Until they are tested, which may be when applying for a job where color differentiation is important, many people never know  that they are color-blind see different numerals displayed in the pates from those seen by people with normal vision. Color blindness has no cure, but a special contact lens can help victims distinguish red-green color vision, but makes green appear darker than red, enabling the wearer to spot the difference.

Color blindness is usually detected by test with colored plates viewed in natural daylight. In the top plate, people with normal vision see a figure 8; those with red-green defect see 3, the totally color-blind cannot see a numeral. In the middle plate, the normal see 16; those with color defects see no numeral or a wrong one. In the bottom plate, the color-blind see a line windings its way from one x to the other

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