Why do cats' eyes glow in the dark?

Because they possess an imageintensifying device at the rear of their eyes. This is a lightreflecting layer called the tapetum lucidum (meaning literally 'bright carpet'), which acts rather like a mirror behind the retina, reflecting light back to the retinal cells. With this, the cat can utilize every scrap of light that enters its eyes. With our eyes we absorb far less of the light which enters them. Because of this difference cats can make out movements and objects in the semi-darkness which would be quite invisible to us. Despite this efficient nocturnal ability it is not true that cats can see in complete darkness, as some people seem to believe.

On a pitch black night they must navigate by sound, smell and the sensitivity of their amazing whiskers.