Lenses

A lens is a specially shaped piece of glass, plastic or other transparent material that is used to give an image. Even a drop of water can act as a lens.

Lens types
The five main types of lens are drawn below:

We usually refer to lenses as either convex or concave. A lens that is convex on both sides should really be called bi-convex and one that is concave on both sides is bi-concave. The meniscus lens behaves as either a convex or a concave lens depending on which side is the more sharply curved.

You can easily see the difference in the behaviour of convex and concave lenses. If you hold a convex lens in front of some writing then the writing will look bigger, but through a concave lens it will look smaller.

Images with lenses
Both types of lens produce images but they may be of different types. You can show this easily by trying to focus an image onto a piece of paper. With a convex lens you will get an upside down real image but with the concave lens the image is virtual.

Convex lens

Convex lenses converge or concentrate light to a focus if the image is further from the lens that its focal length.

What is it like to look through?
1 Close to object - it magnifies.
2 Far from eye - image upside down.

Uses of convex lenses  Eye, camera, overhead projector, focus sunlight, projector
microscope, simple telescope, glasses (to correct for long sight), magnifying glass

Concave lens

Concave lenses diverge or spread out the light. There is no real focus. Image is always virtual. Power is negative.

What is it like to look through?
The image that you see is always the right way up and smaller.

Uses of concave lenses  Glasses (to correct for short sight), spy holes in doors,
some telescopes, back window of coaches