# Diffraction

When you watch waves entering a harbour you may see them spreading out after they go through the gap in the harbour wall
Television signals are much more difficult to receive in hilly areas than radio signals
You may see a halo of colour around street lamps on a foggy night
CDs and DVDs will show beautiful rainbow patterns if held up to the light
If you are in a room with the door open you may be able to hear someone speaking outside although you cannot see them
If you look at a street lamp through an umbrella you will see an interesting pattern of coloured dots

All these effects are due to a property of all waves called diffraction.

Diffraction is the bending of waves round an obstacle or through a hole.

You can show diffraction effects with water waves in a ripple tank, microwaves, sound and even light. The patterns for some water wave diffraction are shown in the diagrams.

They show that:
(a) the diffraction effects are greater for long wavelength waves

(b) the diffraction effects are greater for small holes

The next diagram shows a wave bending round the edge of a barrier. This explains why it is possible to hear sounds and receive radio signals even if there is something between you and the source of the waves.

If the wavelength of the waves is shorter the spreading, diffraction, effect is much smaller as well. This explains why television waves are much more difficult to receive in hilly areas than radio waves which have a longer wavelength and why the diffraction of light is so difficult to observe.

Its very small wavelength (about 600 nanometres (600 thousand millionths of a metre) for yellow light) is also the reason why diffraction of light is only big enough to be observed with very small obstacles. The fact that it can be seen at all is very good evidence for light being a wave.

WORD VERSION AVAILABLE ON THE SCHOOLPHYSICS CD