I got this from http://www.bigcats.org/abc/identification/catseyes.html
Why do cats eyes glow in the dark?
Question: Can you tell me why cat's eyes glow in the dark.
Thank you for your help.
Cat eyes don't actually glow in the dark. They do appear to be bright green, most of the time, when they reflect light from another source. A way of thinking about this is to imagine a mirror. It doesn't glow in the dark, either -- but if you shine a light on it, the light is reflected back to you and you see it as a light coming from the mirror. Reflectors like you see at the end of driveways or on bicycles are another example of something that doesn't actually glow in the dark but that shines brightly when it reflects light.
If you go into a dark room without a light and look for your cat, you can't find it by looking for its eyes. This proves that they don't have glow all their own. It is pretty easy to find a cat in a dark room with a flashlight, though. Even if you can't see the cat very well, you can see the reflection from its eyes.
The portion of the cat's eyes that reflects light is the retina, or more specifically, the tapetum of the retina. Most cats have green tapetal reflections because they have colored retinas. Many Siamese cats do not have pigmented (colored) retinas. When light shines in their eyes, the reflection is red, because you see the blood vessels in the retina instead of the green color that hides the blood vessels in other cats.
The advantage of having reflective retinas is that light that hits the retina passes through the rods and cones (the part of the eye that allow vision to occur) first. Some of it is absorbed and processed so that the cat can see. The rest passes into the cells behind the eye. The reflective layer in a cats eye reflects the light that would have been lost in this manner back through the rods and cones, giving the cat a second opportunity to use the light to see. This helps cats to see better in the dark than animals that don't have a reflective tapetum.
I don't know for sure, but I think that people must not have much color in their retinas, which is probably what produces the "red eye" in photographs.
Some dogs have blue tapetums and so their eyes appear to have a blue reflection when light shines on them.
I am not sure what the function of the different colors in the retina really is. I will try to see if I can find some information on this for you.
I don't have any photographs that show the green reflection except in textbooks.
I hope this helps you a little.
Mike Richards, DVM