Source: www.wikipedia.org 

The Red-Eye Effect in photography is the common appearance of red eyes on photographs taken with a photographic flash when the flash is too close to the lens (as with most compact cameras). This is NOT to be confused with an actual red coloration of the iris.

The light of the flash occurs too fast for the iris of the eye to close the pupil. Light passes through the blood-rich area alongside the iris (called the choroid) and then strikes the retina. Some of the light is reflected back out through the iris. The camera records this reflected light which has now passed twice through the reddish choroid. This principle is used in fundoscopy, an examination of the retina with an opthalmoscope, wherein a positive reddish-orange reflection is a normal finding.

The effect is generally more pronounced in people with gray or blue eyes and in children. This is because pale irises have less melanin in them and so allow more light to pass through to the retina. Children, despite superficial appearances, do not have larger pupils but their pupils are more reactive to light and are able to open to the fullest extent in low light conditions.

If photos or videos are shot with infrared-sensitive equipment, the eyes also usually look unnaturally bright. The reason is the same: the blood-rich retina reflecting light into the camera. 

In many species the tapetum lucidum, a light-reflecting layer behind the retina which improves night vision, intensifies this effect. This leads to to variations in the colour of the reflected light from species to species.