[Intro]

It can be terrifying to wake up in the middle of the night to a pair of glowing eyes staring at you from the darkness, only to realize that it's just your cat...or is it?

Lots of animals, including alligators, fruit bats and dogs, all have a shiny structure in their eyes that gives them better night vision, which also causes this creepy glow.

The layer of tissue at the back of your eyes is called the retina and it's made up of special, light-sensitive cells. Light that hits those cells gets turned into an electrical signal that's sent along the optic nerve, which sits behind the retina. That signal travels all the way to the brain. And light that misses those cells isn't turned into a signal, so your brain can't detect it.

Many species across the animal kingdom have this same, basic eye structure, from reptiles to birds to other mammals, like cats. One way animals have evolved to see better is to have more of these light sensitive cells in each eye. Humans, for example have over 95,000,000 of them per retina. But a lot of animals, especially nocturnal animals whose eyes have to work well at night, have another way of detecting more light.

They have a structure called the "tapetum lucidum", which is Latin for "bright tapestry." It's a layer of tissue that sits behind the retina and acts like a mirror. The tapetum lucidum reflects light that goes through the retina back at those light sensitive cells, giving them another chance to detect it. But some of that reflected light flies back out of the animals' pupils, which is what makes it seem like their eyes are glowing.

The color of that glow, also called "eyeshine," depends on what the tapetum lucidum is made of. And different critters have different highly ordered molecules or fibres that create a reflective surface. Some fish use guanine, one of the chemical building blocks in DNA and have white eyeshine. Sheep have collagen, which also provides structure to muscle and skin in lots of animals, which leads to a blue or green glow. And cats use riboflavin and zinc, among other molecules. And the amount of zinc does a lot to determine how yellow or green or even blue their eyeshine is.

So your kitty's eyes aren't actually like little flashlights. They're just reflecting some of the ambient light in your room. But if they start glowing red and shooting laser beams you should probably run.

Thanks for asking. And thanks to all of our patrons on Patreon who keep these answers coming. If you'd like to submit questions to be answered you can go to patreon.com/scishow. And don't forget to go to youtube.com/scishow and subscribe.

[Outro]