Mirrors play throw and catch with light. They catch light from in front of them and throw it back the way it came. Mirrors are made from thin metal sheets inside glass, but many other surfaces reflect light as well. A smooth lake mirrors the sky above it, and you can often see your face in shiny shoes or a polished spoon.
Light rays stream in straight lines, so a flat and smooth (plane) mirror reflects things much as they are. The reflected image looks like the original because the incoming rays bounce back in parallel lines. However, a mirror that curves inward (concave) makes things look bigger, while one that curves outward (convex) can make them look smaller.
Mirrors reflect heat as well as light. Hot objects give off infrared radiation, which is like invisible, hot light. When infrared hits a mirror, it reflects straight back again. You can test this effect for yourself by wrapping some silver foil around your arm. It feels warm because your body heat is being reflected back.
Cats are like walking mirrors. They can see at night because they have special reflecting surfaces (miniature mirrors) behind their eyes. These catch incoming light and bounce it back out through their eyes again. The light passes through their eyes twice, and this gives them double the chance to see dim objects. That’s why a cat’s eyes shine at night or in dim light.
Some telescopes need giant mirrors, but if a mirror becomes too big, it bends and buckles.
To get around this, the biggest and best space telescopes use mirrors split into dozens of honeycomb-like segments. Bolted onto a framework, very close to one another, they work together like a single giant mirror.