What do lenses do to Light?

Convex Lenses

When light enters or leaves glass at an angle it changes direction. We call this refraction. When light passes through a prism it is refracted. The different colours in visible light are refracted by different amounts and so this produces a visible spectrum of colours.

A lens is a piece of glass shaped so that it either brings rays together (converges) are spreads them out (diverges). Lenses, like curved mirrors, also have a principal focus. For a convex lens rays from a distant object, i.e. parallel rays, would converge at this point.

The position and nature of the image produced by a convex lens depends on the position of the object. You could learn the information in the table below or, if you are competent at drawing ray diagrams, figure it out for yourself.

Concave Lenses

A concave lens makes rays of light diverge. The image it produces is always virtual, diminished and upright, just like for a convex mirror. Think of a concave lens as a rubbish magnifying glass.

People usually wear glasses because the lenses in their eyes either bend light too much or not enough. We can use lenses to correct this.

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Revision Questions

1.       Explain how rainbows are formed

2.       A lens is used to start a fire using radiation from the Sun. What kind of lens?

3.       What does the real image fall on a) in a camera?   b) from a projector?

4.       A convex lens has a focal length of 7cm. A candle is placed 20cm from it. Describe what kind of image will be produced.

5.       Try drawing ray diagrams for each of the 3 object positions in the table above.

6.       A person is short sighted. What kind of lenses would be put in their glasses?