Have you ever walked around a dark corner only to be surprised by glowing eyes staring back at you? The glowing eyes of a cat at night can sometimes be shocking and even a little scary if unexpected. Ancient Egyptians believed cats captured the glow of the setting sun in their eyes and kept it safe until morning.
Ancient Greeks believed there was a light source inside the eyes that was like a gleaming fire. We now know that cat’s eyes appear to glow because they, along with the eyes of many other nocturnal animals, reflect light.
All eyes reflect light, but some eyes have a special reflective structure called a tapetum lucidum that create the appearance of glowing at night. The tapetum lucidum (Latin for “shining layer”) is essentially a tiny mirror in the back of many types of nocturnal animals’ eyeballs.
It basically helps these animals see super-well at night. It is also what causes the glowing eye phenomenon known as “eyeshine.”
How Does It Work?
When light enters a cat’s eye, it can take a few routes. Some of the light directly hits the retina, a layer at the back of the eyeball containing cells that are sensitive to light. These photoreceptor cells trigger nerve impulses that pass via the optic nerve to the brain, where a visual image is formed.
Some of the light passes through or around the retina and hits the tapetum lucidum. The tapetum lucidum reflects visible light back through the retina, increasing the light available to the photoreceptors. This allows cats to see better in the dark than humans.
In the last route, some of the light that bounces off the tapetum lucidum, misses the retina, and bounces back out of the cat’s eyes. This reflected light, or eyeshine, is what we see when a cat’s eyes appear to be glowing.
Do Humans Have a Tapetum Lucidum?
Though our eyes have much in common with cats’ eyes, humans do not have this tapetum lucidum layer. If you shine a flashlight in a person’s eyes at night, you don’t see any sort of reflection.
The flash on a camera is bright enough, however, to cause a reflection off of the retina itself. This is the infamous “red-eye” in photographs. What you see is the red color from the blood vessels nourishing the eye.