Red-eye pictures, on today's Moment of Science.

             These snapshots are otherwise normal, except for an unearthly red color glowing from the pupils of your loved ones' eyes. Are Grandma and Uncle Felix possessed, or is there some other explanation for this red-eye phenomenon?

            Actually, that pinkish red color is always there.  It's the natural color of one of the layers in the back of your eye.

            You can't usually see this color because of work done by special muscles around the pupils.  These muscles cause the pupil to shrink in bright light, allowing less light through to the sensitive inner part of the eye.  In dimmer light, these muscles relax and the pupils expand.  You can see this in action by standing in front of a mirror in a dim room.  Turn on a bright light, and you'll be surprised how quickly your pupils shrink in response.

            Because Grandma's pupils are so good at only letting a certain amount of light into her eyes, it's almost always too dark in there to see the pinkish red inner eye.  However, a camera flash in an otherwise dim room is much too quick for anyone's pupils to react to.  Light from the flash pours to the back of the eye before the pupils can dim it, and the camera lens, pointed directly into the eyes as well, picks up the red color.

            Cameras offering "red eye reduction" give off one or several pre-flashes before the actual flash.  These pre-flashes give Grandma's pupils a chance to shrink before the picture is taken.  Just remember to keep her smiling until the final flash though, because that's what will be recorded!