A contact lens is a tiny piece of plastic or glass that rests on the front of the eyeball. It bends light rays before they enter the eye in ways that help the wearer to see more clearly.

The two main types of lens are called concave and convex. A concave lens is thin in the middle and thick at the edges, so it seems to “cave” inwards. It makes light rays bend outwards, or diverge. A convex lens works in the opposite way. It is thicker in the middle and thinner at the edges. Light rays passing through a convex lens bend inwards, or converge.

A lighthouse must send a long beam of light far out to sea. To do that a very large and heavy lens would normally be needed. Instead, lighthouses use a specially shaped Fresnel lens. It has steps like a staircase, each of which helps to bend the light into a single, powerful beam. A Fresnel lens can be made from glass or lightweight plastic.

By shining a powerful beam of light through a transparent photographic slide, a projector can make a much larger image on a wall. The little image on the slide is shined through a concave lens, which spreads the light rays outwards. The further away the projector is from the wall, the bigger the image becomes.

To magnify means to increase in apparent size. If you look closely at a magnifying glass, you can see that it is a large convex lens, thicker in the centre than at the edges. When you hold a magnifying glass over an object, it makes light rays from the object seem to come from a nearer point, causing it to look bigger than it really is.