You're driving along a lonely road at night. Ahead, in the dark, you see a pair of bright, disembodied eyes. You get closer and the eyes slip away into the grass.
They were just the eyes of a prowling cat. The cat's eyes shone because your eyes, your car's headlights, and the cat's eyes were nearly in a straight line.
The light from your headlights, or the sound from your engine, attracted the cat's attention. The cat looked toward your car. Its eyes focused on your headlights.
Cats' eyes are different than our own. That green glow (although some breeds do have different coloring) you see is because the light is reflecting off a part of their eyes called the tapetum lucidum.
It's a reflective surface that is behind the cat's retinas. Many other animals have versions of it. In cats, the tapetum lucidum increases their sensitivity to light.
A Second Chance
When a cat sees light like demonstrated above, the tapetum lucidum allows any light not absorbed by the retina to be reflected back. We see it as an eerie green in most breeds of cats. For cats, it's an opportunity for them to see the light again and let their brains process it.
People do not have a tapetum lucidum. The red eye glow you sometimes see in photographs is light being reflected from a person's retinas which are red from all the blood vessels back there. (And if you're interested in learning more about parts of the human eye, you can click here to read about the fovea, the part of the eye that lets you read and see other fine details)
Sources And Further Reading:
- "Feline Vision Problems: A Host of Possible Causes." Cornell University College of Veterinary Medicine. Accessed January 17, 2018.
- Inglis-Arkell, Esther. "The Science Of How Eyes Glow In The Dark." io9.com. July 4, 2014. Accessed January 17, 2017.
- Seabrook, Andrea. "Why Do Animals' Eyes Glow In The Dark?" NPR: Science Out Of The Box. October 31, 2008. Accessed January 17, 2018.
- Shaw, Ethan. "What Animals Have A Tapetum Lucidum?" Sciencing. April 25, 2017. Accessed January 17, 2018.
The podcast episode is a rerun from many years ago. The text post above has been updated to include the scientific terms and information from more recent sources about why cats' eyes look like they're glowing at night.