Why do dogs' and cats' eyes reflect light? Does bright light affect their vision?

Question

At night when the light is pointed towards dogs and cats, their eyes give a gleaming effect (greenish light). It even happens during a flash of a camera. I've seen this on a cat and the dog and afterward both stared for a while.

I don't flash lights into my animal's eye on purpose, but while finding my way in darkness I need to watch my step. Even when the light falls on their eye they do not turn away or move their head other way they simply stare on. Is it disturbing for them to have the light shine on their eyes? Does this have any effect on their vision, or is it just a temporary effect which fades (as in humans' vision when exposed to a flash)?

Answer 1

It is called tapetum lucidum and it works similarly for many (nocturnal) mammals. Humans and most primates don't have it, cats and dogs do.

It is a tissue in front of the retina. It lets the light go through but reflects the light coming back from the retina, sending it back to the retina and thus increasing the amount of light available for (night) vision.

For the second part of your question, although the details of night vision are different between humans and dogs/cats, the pupil dilates/contract in the same way as it does for us, so shining light at their eyes at night will have the same affect as it does for us.

I think that staring at the light source is more like a "fear freeze" effect.

There's a nice collection of photos on Wikimedia Commons.