Have you ever looked out into your backyard at night and wondered how many spiders are lurking out there? If you have a flashlight, you can spot them by the creepy green glow of their eyes.
When light shines into the eye, it reflects off the tapetum and back onto the retina. More light hitting the retina means that the animal can get the most out of every photon, improving its ability to see in the dark. The reflected light also creates what’s called “eyeshine,” which is why cats’ eyes seem to glow in the dark.
It’s also why you can see spiders’ eyes glowing bright green in the night, if you know how to look: light reflects off their tapeta, and they’re looking in your direction. Enjoy thinking about that.