Have you ever wondered what causes eyes to appear red in photos? Usually this happens indoors or at night when the pupils are more dilated than usual. The light from camera flashes are generally located only a few inches from the camera's lens. When the flash goes off, light goes straight into the pupil and gets reflected off the back of the eye, and straight in to the camera lens. The effect is magnified with dilated pupils - that's why the "red eye" mode of cameras involves a pre-flash before the main flash so that pupils become smaller before the real picture is taken and red eyes are minimized.
Why do eyes appear red? As light from the camera flash enters the eye, it's reflected back into the camera lens. The retina (the nerve tissue in the back of the eye) is essentially transparent, but the underlying choroid (the vascular layer of the eye, containing connective tissue, lying between the retina and the white of the eye) has pigment and a lot of blood vessels. It's the choroidal blood that gives the reflected light coming out of the eye its reddish hue, hence the appearance of red eyes.
There are ways to minimize the occurrence of red eye besides using the red eye mode for your camera's flash. Reducing red eye can actually be achieved by using a little bit of physics. If red eye is caused by the flash being sent in a straight line in to the eye and then directly reflected back out, you can decrease it by having the flash source at an angle so less of it will hit your camera's lens. Using a detached flash that's located several feet away from your subject will do the trick as well as reflecting the flash off the ceiling.