|Image credits latmario|
Humans do not have this tapetum lucidum layer in their retinas. When we light up the eyes of a person with a flashlight at night, you can not see any sort of reflection. However, the flash of the machine is light enough to cause a reflection in the retina - what you see is the red color of blood vessels that nourish the eye. When you take a picture with the flash sometimes we get a red eye. The red color comes from light that is reflected in the retinas of our eyes. In many animals, including dogs, cats and deer, the retina has a special reflective layer called the tapetum lucidum (inside the eye that reflects light that passes through the retina in the eye, causing it passes twice through the retina), which works almost like a mirror on the back of the eye. If illuminate the eyes of animals with flashlights or headlights at night, they reflect a white light.
The existing cameras have features that reduce the effect of "red eyes", before firing the flash lamps in order to retract the pupils. Another solution is to look at a bright object a few moments before taking the picture, what causes the contraction of the pupil.
Translated from: Quarks e Gluões