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Why do dogs eyes glow in the dark?

They have a reflective surface behind the retina (light sensitive back surface) of the eye which bounces light from, for example, your headlights back at you somewhat like highway reflectors. The purpose of the surface is to increase the ability of the retina to detect faint light -- that is, the ability of the animal to see in the dark -- by making the light pass *twice* through the retina. Humans don't have this surface, but very bright light will bounce back from the retina itself enough to be seen. Since the retina is red (from its ample blood supply) you see a red reflection, and this is the source of the "red spot" in the eyes in photographs of people who looked at the flash.

Christopher Grayce

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