Photographs with red-eyed subjects are becoming a thing of the past because cameras are flashing twice. What causes red eye? How can a double flash prevent it?
The pupils in your eyes are really holes covered with see-through skin. Objects appear red in color because they reflect red light. The back of your eye reflects red light because it has many blood vessels.
When you are photographed, the light from the camera's flash enters the eye through your pupil and some of the light bounces back toward the camera. Your eyes look red in a photo when your pupils are dilated (enlarged). With a larger opening, more light enters the eye and more red light is bounced off the back of the eye.
When the camera flashes, the light entering your eye causes your pupil to involuntarily contract. This means that the pupil opening gets smaller. But the speed of light is faster than is your eyes' response, and the red light that bounces off the back of your eye exits your eye before the pupils can contract. If the camera flashes twice, the first flash causes the pupil to contract. With the pupil opening being small, only a small amount of light from the second flash enters the eye. There is still some red light reflected to the camera but rarely enough to result in "red eye."
Discover for Yourself
The pupil in your eye gets smaller in bright light and larger in the dark. Here's how to see light change the size of your pupil:
* Sit in a brightly lit room or sit outside in the sunshine. Caution: Never look directly at the sun because it can permanently damage your eyes.
* Close one eye and leave the other eye open. 
* Cup one hand over the closed eye or use an eye patch.
* After 2 to 3 minutes, look at the pupil of the open eye in the mirror.
* Then, open the closed eye and quickly look at its pupil in the mirror.
The pupil of the eye that remained open will be smaller than the pupil of the eye that was closed (covered).
At night when light hits the eyes of animals they do not look red. Instead they seem to glow. To find out why this happens, see EYES: GLOWING.
For more information about your eyes, see Janice VanCleave's Human Body for Every Kid.
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