What causes 'red eye'?

It is almost always surprising for laymen to find out that the pupil isn't really a thing of substance - it's just a hole that appears black because so little light reflects off it. (The 'pupil' can't afford to let too much light through - see here.) Yet, take a photograph and our pupils glow red. Why?

A pretty useful clue in solving this problem comes when we realise that a camera's flash, or some other such bright light source, is almost always needed to generate 'red eye'. Of course, what is happening is that the flash forces so much light through the pupil that some of it is obliged to get reflected back, providing us with the colour of the retina at the back of the eye.

Why is it red? That's a sore point. While our eyes are exquisitely designed in many ways, they do suffer from a few construction flaws, and one of them is that the retina's blood vessels lie in front of the retina's light-collecting cells. This means that light from the outside world has to pass through a thin layer of blood vessels before it can be detected by our photoreceptors, which in turn means that the image quality is (slightly) degraded. (An animal like the squid, you'll be pleased to know, has the arrangement of its layers the correct way around.) So the red that you see in 'red eye' is largely from the blood vessels that cover the retina. You just don't normally see the 'pupil' as red because not enough light enters it.