Short · Light and eyes
Why do eyes glow red in flash photos?
That red glow is the inside of the eye, lit by the flash and sent straight back. Slide the flash away from the lens, or shrink the pupils first, and the red disappears.
Try it yourself
Opens Red-Eye Lab. Change the same thing yourself.
Transcript
Why do eyes glow red in flash photos? You're photographing the inside of the eye. In dim light, pupils open wide, and a flash is too quick for them to shrink. Light goes in, hits the blood-rich back of the eye, and bounces out red, straight back toward its source. Now slide the flash away from the lens. The red glow misses the camera. Some cameras fire a quick flash first, so your pupils shrink before the photo. Try it yourself at Explainer Tools dot com.