Mirrors Make Cats' Eyes Shine in the Dark

Q: Why do cats' eyes glow in the dark?
Shirley, Panama City, Florida

A: A cat's eyes bounce back part of the incoming light like focused reflectors. A lens and curved mirror inside the animal's eye return light — shaping it into a cone and sending it back, nose on, to the light source. Cats' eyes are like bicycle taillights but not for the same reason. Cats get no competitive advantage from shining eyes. Instead their reflectors make better use of dim light, when they hunt at night.

Humans' eyes don't have a similar mirror but many animals do: whales, dolphins, horses, hippos, antelopes, dogs, rats.

Eyes of animals with backbones evolved in a weird way — as an outgrowth of the brain — so eyes are functionally inside out. Nerves inside the eye that send messages to the brain are really part of the brain. And that causes a clutter problem in the eye. Unlike cameras, with their clean open space between shutter and film, a tangle of structures crowds the space between pupil and retina--the light receptors. Light has a tough job getting through this jungle. Only about ten percent makes it to and gets absorbed by the retina — the film in a camera.

That's why cats have mirrors in the back of their eyes. The mirrors give the retina a second chance to capture photons it missed on the first pass. What's more, the mirror is close to the retina so less intervening clutter absorbs the reflected photons.

The more light the retina absorbs, the better the cat sees. The better the cat sees in dim light, the better it hunts at night. That's why cats' eyes are like bicycle taillights. To hunt and eat better.

Further Surfing:

Washington State University: Cats eyes anatomy

Wild cat facts, Facts & Figures