Not only cats’ but the eyes of some other nocturnal animals like leopard and tiger also show their distinctive glow at night, though the familiar example is that of a cat. During the day when there is dazzling bright light the cat spends most of the time dozing in some cozy corner. (Man sleeps for about 8 hour in a day on an average but the cat sleeps for about 16 hours – spending only 1/3 of its life awake.) Even if it is awake during day pupils of its eyes narrow to a slit to avoid dazzling bright light. They widen in dark to allow maximum light to enter.
A cat cannot see at all in complete darkness but it can make the best use of whatever little light that is available. Rays of dim light entering the eyes through pupils are gathered on retina which has two kinds of cells – rods and cones. Compared to humans, the cat has relatively more rods than cones in its retina. Rods provide sensitivity to low light, while resolving power and color are derived from cones. The functions differ, but together they add up to what we call vision. Besides rods and cones the cat has a glossy area behind the retina. Known as tapetum lucidum, it works like a mirror and provides the retina a second chance to gather the light rays which it missed earlier. Humans are not so fortunate, for any light that passes unabsorbed through the retina is lost for vision. In cats, such light is reflected back to the retina by the lustrous tapetum lucidum. This makes cat’s eyes glitter at night.
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