The eyes of many species of vertebrates appear to glow at night because of the reflection of light in the eye across a thin membrane called the tapetum lucidum. Located directly behind the retina, this membrane enhances animals' vision in night or low-light conditions by reflecting light back through the retina.
At night, humans can use flashlights to spot animals that possess tapetum lucida by causing a phenomenon called "eye shine." Eye shine is the iridescent glow observed in animals when light is shone into their eyes, which floods the eye with light that is then reflected by the tapetum lucidum. Depending on the species, the angle at which the light is pointed and the relative angle of the human observer, the color of the glow can be red, green, blue or white.
The tapetum lucidum is especially beneficial to nocturnal land animals such as raccoons, spotted hyenas, African lions, gray wolves, bats and badgers, which rely on the cover of night to hunt their prey or scavenge for food. The tapetum lucidum is also present in sea animals such as sharks, skates, rays and squid, which enables the animals to see at night and, in deep sea hunters, extremely low light conditions.