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They DON'T!!!!

They REFLECT light,dear.

When light enters the eye, it's supposed to hit a photoreceptor that transmits the information to the brain, Powell explains. But sometimes the light doesn't hit the photoreceptor, so the tapetum lucidum acts as a mirror to bounce it back for a second chance.

A large number of animals have the tapetum lucidum, including deer, dogs, cats, cattle, horses and ferrets. Humans don't, and neither do some other primates. Squirrels, kangaroos and pigs don't have the tapeta, either.

And not all eyes animals' glow the same color. Powell says this is due to different substances — like riboflavin or zinc — in an animal's tapetum. "Also," she says, "there are varying amounts of pigment within the retina, and that can affect the color." Age and other factors also can change the color, so even two dogs of the same species could have eyes that glow different colors.

That is the way their eyes reflect light. It's an adaptation for improved night vision.