Peters' Epauletted Fruit Bat (Epomophorus crypturus), Kruger National Park - photo by Peet van Schalkwyk
Have you ever noticed how under certain lighting conditions some animal's eyes seem to glow? Animals that are nocturnal hunters - and a few of them that are not - have something called eyeshine. Eyeshine is the light that we see reflected back from the animal's tapetum lucidum (a membrane behind the animal's retina). Light enters the eye, passes through the retina, strikes the reflective membrane, and is reflected back through the eye toward the light source. This phenomenon makes the most of what little light there is at night for these nocturnal creatures.
Humans can display the red-eye effect in flash photography, but we do not have a tapetum lucidum, and thus, do not have eyeshine.
Eyeshine is best observed by wearing a head lamp or holding a flashlight at eye level against your temple because the light is reflected right back into your line of vision. In the wild, many times, if not for this eyeshine, an animal would otherwise go undetected by us at night.
Many biologists can identify animals in complete darkness based on the color of the eyeshine. Cats, deer, and raccoons are typically yellow and green; alligators are typically red-orange; and spiders are said to be "sparkling". It is not known why certain kinds of animals have a certain eyeshine color.
Other eyeshine-related characteristics that help identify an animal include the distance between the pupils, the manner of blinking, the movement of the eyeshine, and the height at which it is coming from.
"Eyeshine" (Texas Parks & Wildlife Young Naturalist)
photos of "Wild Animals With Glowing Eyes" (The Firefly Forest blog)