Why cat eyes shine in the dark

A cat's eyes can seem to glow when a beam of light hits them. The effect comes from the tapetum lucidum, a reflective layer behind the retina.

The tapetum sends some light back toward the retina, giving the light-sensing cells another chance to absorb it. 1 That extra pass helps cats detect light and movement in dim conditions. 2 The same reflection can travel back out of the eye, where a person sees it as eyeshine. 3

Cats are reflecting light, rather than producing it. A flashlight, camera flash, or car headlight can reveal the effect because the light source and the observer are close to the same line of sight.

Sources: Tapetum lucidum · VCA: Why does your pet's eyes glow in the dark? · Indiana Public Media: Why Do Cats' Eyes Glow At Night?

Image material: Black cat with yellow eyes