Veterinary ophthalmologist J. Phillip Pickett of the Virginia-Maryland Regional College of Veterinary Medicine offers the following explanation:
"Red eye," the all too familiar nemesis of amateur photographers, occurs when a person looks directly at the camera when his or her picture is taken. If the flash is on the same axis as the visual axis of the camera, the reflection of the light off the blood vessels in the person's retina will give an eerie, satanic "red eye" look. People with light-colored eyes usually exhibit the worst red eye effect; those individuals with dark-colored eyes may have enough pigment in the back of their eyes to mask this so-called red reflex.
Dogs, cats and almost all domestic animals have a special reflective layer in the back of the eye termed the tapetum, which enhances nocturnal vision. Light passes through the animal's retina from outside of the eye and is then reflected back through the retina a second time from the reflective tapetal layer beneath the retina. This double stimulation of the retina helps these species to see better than humans do in dim light situations. The color of this tapetal layer varies to some extent with an animal's coat color. A black Labrador retriever, for example, will usually have a green tapetal reflection. A buff Cocker spaniel will generally show a yellow tapetal reflection. Most young puppies and kittens have a blue tapetal reflection until the structures in the back of the eye fully mature at six to eight months of age. "Color dilute" dogs and cats, such as red Siberian huskies and blue point Siamese cats, may have no tapetal pigment, and may therefore exhibit a red reflex just like human beings.
Answer originally posted on May 29, 2001.