Why do cats'
eyes glow when
the lights go off?
Can't play sound? The illustrated version is right below.
Read the transcript
The full narration of the video.
A cat's eyes don't make any light at all. You get up at night and switch on your phone's flashlight. Two green lamps light up under the sofa. But turn off every light, leave the room pitch black, and those two lamps go out too.
They aren't lamps. They're reflections. So let's send a beam of light on a trip through a cat's eye. It passes through the clear front of the eye, and gets focused onto the very back.
Back there sits a layer of cells that catch light. A cat only sees the light those cells catch. In the dark, there's little light to begin with, and plenty of it slips straight past those cells. But behind those cells, a cat has an extra shiny layer.
Light that slipped past hits it, bounces back, and passes the cells a second time. The same light gets used twice. That's how a cat sees things in the dark that we can't.
Though with no light at all, a cat can't see either. That layer works like the reflector on the back of a bike. A reflector has no light of its own. Hit it with headlights, and it sends the light straight back.
And the driver sees a bright spot. A cat's eye sends light back the same way it came. Hold the flashlight next to your face, and the returning light lands right in your eyes.
Someone standing off to the side sees a much dimmer glow. Its real name is the tapetum lucidum. It's a thin layer at the very back of the eye. In cats it's packed with neat rows of tiny particles, made with vitamin B2 and zinc.
The reflector is the tapetum. The headlights are your flashlight. And the driver's bright spot is the pair of green lamps you saw. The color varies from cat to cat, mostly green or yellow.
Some blue-eyed white cats have very little of it. A camera flash shows the red of blood vessels instead. People don't have one at all, so the red-eye in your photos is that same red. That second bounce has a cost. The light spreads a little, so cats see less sharply than we do.
They traded sharpness for seeing in the dark. In 1934, a man in England saw a cat's eyes flash by the road, and patented the reflective studs in the middle of the road. To this day, they're called cat's eyes.
Where else have you seen a pair of glowing eyes at night?
Two green lights
under the sofa
A cat's eyes
make no light at all.
In short: they aren't lamps. They're reflections.
The same light,
used twice
That's how a cat sees things in the dark that we can't. But with no light at all, a cat can't see either.
Like the reflector
on a bike
A reflector has no light of its own. When headlights hit it, it sends the light straight back the way it came, so the driver sees a bright spot. A cat's eye does the same. Hold the flashlight next to your face and the returning light lands right in your eyes. Someone standing off to the side sees a much dimmer glow.
Its real name is the tapetum lucidum, Latin for "bright carpet." It's a thin layer at the very back of the eye, right behind the light-catching cells. In cats it's packed with neat rows of tiny particles made with vitamin B2 and zinc.
Green, yellow,
and sometimes red
The extra bounce
has a cost
sharply than we do Light spreads a little on the second bounce. Cats traded sharpness for seeing in the dark, which is why they're so lively at night (why cats sprint around at night).
are called
"cat's eyes"
Quick check
Why do a cat's eyes glow when a flashlight hits them in the dark?
Like a bike reflector: it has no light of its own and just sends light back. In total darkness, nothing goes in, so nothing glows.
