Originally posted by wolfsoul
Some animals have a reflective layer on their retinas that kind of mirrors the backs of their eyes. People don't have it. But the flash of a camera is so bright that you can see the reflection of the retina, and the red is the colour of the blood vessels. I can't remember, but the pupil having to dilate really quickly also causes the eyes to look significantly red.
How the heck did you know that..LOL