Why Do Cats’ Eyes Shine in the Dark?

The Answer

Behind the light-sensitive retina at the back of a cat's eye is a shiny layer that works like a mirror. Your torch light is bouncing off it.

The layer is called the tapetum lucidum, Latin for "bright carpet". Light entering the eye passes through the retina, where cells called rods and cones detect it. Some of it slips through without being caught. In our eyes that light is simply absorbed and lost. In a cat's eye it hits the tapetum and is reflected straight back through the retina, giving the cells a second chance to catch it. Whatever still isn't caught heads back out of the eye, towards the torch, and that's the glow you see.

It's a brilliant adaptation for an animal that hunts at dusk and at night: cats can see in far dimmer light than we can. Dogs, deer, foxes, cows and crocodiles have a tapetum too, which is why their eyes shine in car headlights. The price is slightly blurrier vision, because the bounced light scatters a little.

People don't have one. The red eyes in a flash photo come from light reflecting off the blood vessels at the back of the eye, not from a mirror layer.

The principle: Adaptation for night vision. A reflective layer behind the retina sends light back through it for a second chance to be detected, letting night hunters see in very dim light.