Why do eyes turn red in flash photos?

The pupil, the black circle in the middle of the eye, is actually a clear opening. Light has to pass through this opening and reach the retina at the back of the eye for us to see. The retina and the layer behind it have many blood vessels, so they look reddish.

When flash light goes straight into the pupil, hits this red inner surface and comes back to the camera, the pupil shows up red in the photo. When you look into someone's eye normally, the inside looks black because almost no light is shining that far in.

In a dark place, the pupil opens wide to take in more light. If the flash fires then, the light rushes in through the wide opening and back out before the pupil has time to shrink. That is why red eyes show up more often in photos taken indoors at night or in front of birthday candles than in bright daylight.

The red-eye reduction feature on cameras uses this. It fires the flash once first to make the pupils smaller, then takes the picture with the second flash.

If you photograph a dog or cat with a flash at night, its eyes flash green or yellow instead of red. These animals have a mirror-like layer in front of the retina that sends light back, so they can use the light a second time in the dark.

What glows in the photo is this reflective layer. Many animals have it, such as dogs, cats, raccoons, cows and deer, but humans do not. A cat's eyes flashing in car headlights on a night road is the same kind of reflection.

Sometimes in a child's photo, only one eye glows white. This can happen when the child is looking off to the side of the camera so that the optic nerve inside the eye lines up with the lens, and most of the time it is just this kind of angle issue.

However, if the white glow covers most of the pupil and shows up again in several photos, it can be a sign of a cataract or a rare eye disease. The American Academy of Ophthalmology advises taking a few more photos from different angles and, if it still appears, bringing the photos to an eye doctor.