Why do dogs' eyes shine in car headlights?

Chris: If you look at the back of the eye in animals that go out at night (nocturnal animals) or animals that are likely to be hunted at night they have this structure called a tapidum lucidum which is Latin for bright carpet. If you dissect a sheep's eyeball you'll see this surface that looks like mother of pearl. The back of the eyeball is highly reflective like a mirror.

The reason for having this is that if you're under low light conditions, light comes into the eye, the retina sticks out into the jelly-like stuff in the back of the eye but any light that misses it will be soaked up by the back of the eyeball. By having the back of the eye a bit reflective then any light that missed the retina the first time can be bounced back onto the retina and you have a second chance of seeing it. The payoff is that you can see in low light. The downside is that the light gets altered in its path very slightly so the acuity, how well you can see, is lower with this structure. Dogs, cats, sheep, cows and horses all have it. If you shine a torch at a dog in the dark its eyes look like they're glowing (a bit like Tony Blair on those posters that you saw about ten years ago). It's nothing to do with the pupil being open wider or smaller, although because it's dark, the pupil will be open wider.