1 Answer

Refraction of light.

Eyes contain light-receptors, called rods and cones. Cones allow creatures to see color and sharp detail and function well in the bright light of daytime. Humans are diurnal rather than nocturnal because we have more cones than rods, giving us better functionality —and comfort—during the day. Rods, on the other hand, function well in dim light conditions, and are designed more for light gathering and seeing motion. Nocturnal animals have many more rods than cones.

Light from distant sources such as the moon and stars provides enough light to allow animals to navigate with ease. Thus, nocturnal animals can be hidden under the cloak of darkness but still function in their daily—or nightly—quests for food, mate or shelter.
But why do some animals exhibit the eerie glow of eyeshine while others do not? It all has to do with a special, mirror-like membrane at the back of the eye, called the tapetum lucidum, meaning “bright tapestry.” As light enters the eye, it passes through the retina and is reflected from this tapestry of cells, sending the light back through the retina, providing twice the light as would be available normally. Thus, when a bright light, such as a spotlight or headlights, is shone into the eyes of such a creature, their eyes appear to glow, giving them the spooky eyeshine.

Many animals that move about in low-light conditions have this layer of cells to enhance their vision, allowing them to function quite well in the dark.

As I drove through the gathering darkness, the yellowish eyeshine of a foraging raccoon and the red eyeshine of rabbits were easy to spot, even though their bodies were half hidden in the roadside weeds. It is fun and challenging (and sometimes dangerous!) to identify animals by the color of their eyeshine during nighttime drives through the country.
Spotlighting will point out those animals with a tapetum. Foxes and rabbits eyes glow red; many felids (cats) exhibit a greenish shine; raccoons have yellowish eyeshine; bullfrogs show green. Most owls show red eyeshine. “By their eyes ye shall know them,” although not everyone agrees on the color for each species.

One might think that humans have eyeshine, too, because of the common “red-eye” effect in family photos. This effect is actually the result of a reflection of the many blood vessels in our eyes when pupils don’t have enough time to contract against a bright flash of light. We do not have a tapetum lucidum as our eyes have evolved for daytime function.

Still, humans have a greater capacity for seeing at night than most of us realize. Try going out at dusk, just as twilight is purpling the skies and fields. Walking along a gravel road or a wooded path as darkness settles over the fields and woods is quite comfortable if our eyes are given the time to gradually adjust.

Without using artificial light, even on a moonless night, our eyes develop passable night vision. A chemical called rhodopsin—also known as visual purple—allows us to see better in the dark. Light breaks down this chemical, but with time in the dark, 80 percent of our night vision is recovered in 30–45 minutes. And, thanks to the rods in our eyes, motion is easy to detect in these low-light conditions.

For spotting eyeshine, a simple flashlight is a great tool for an evening stroll. In order not to destroy your night vision, hold the back end of the flashlight to your forehead, shining the light away from you, and look down the beam of light, not into it. Or, for a much easier approach, try a headlamp when exploring fields at night. Keep track of the nocturnal animals out and about by jotting down the colors of eyeshine you see.

Whether you decide to spotlight night life or allow your own night vision to guide you through the evening, many great discoveries await the nocturnal naturalist.
One can illuminate and expand one’s knowledge of natural history while searching for eyeshine, watching the movement of nocturnal animals or just enjoying the thrill of sitting or walking through a starlit forest.

It is an experience not soon forgotten!

Thank wiki.