Why do camera flashes make eyes red and why do two flashes correct this problem?
The retinas of your eyes appear reddish when you look at them with white light. The red eye problem occurs because light from the flash passes through the lens of your eye, strikes the retina (which allows you to see the flash), and reflects back toward the camera. This reflection is mostly red light and it is directed very strongly back toward the camera. The camera captures this red reflection very effectively and so eyes appear red. The double flash is meant to get the irises of your eyes to contract (as they do whenever your eyes are exposed to bright light or you are startled or excited). The first flash causes your irises to contract so that less light from the second flash can pass into and out of your eyes. Unfortunately, this trick doesn’t work all that well.