The red eye effect in photography is the common appearance of red eyes on photographs taken with a photographic flash. The light of the flash occurs too fast for the iris of the eye to close the pupil. The flash light then illuminates the blood-rich retina at the back of the eye, resulting in a red appearance of the eye on the photo.
In many species this effect is intensified by a light-reflecting layer behind the retina known as the tapetum lucidum. This leads to variations in the colour of the reflected light from species to species.
The red eye effect can be prevented by preceding the flash by a period of bright light, allowing the iris to close (the "anti red eye system"), or by having the flash illuminate the person from an angle, thereby avoiding the illumination of the retina.
Retinoblastoma is a cancer of the eye which is often causes the appearance of a "white eye" effect instead of the expected red eye.