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.
Cats' eyes have mirrors in the back of the eyeball. Photo courtesy of
Wikipedia.
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.
(Answered Jan. 30, 2002, updated Aug. 7, 2007)
Further Surfing:
Washington State University: Cats eyes
anatomy
Wild
cat facts, Facts & Figures
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