Answer: You get interference when two wave motions
meet.
You get reinforcement (constructive interference) or destruction
(destructive interference) of two waves when they get 'instep'
or 'out of step'. When the waves are 'in step' (known as 'in
phase') they strengthen each other. When the waves are 'out of
step' (known as 'out of phase') they weaken each other.
The diagram shows the two waves are in phase, crest matching
crest and trough matching trough at the points labelled R. Here
the two waves reinforce each other. If the waves have other frequencies
at other places, marked D, they must be out of phase, with a
crest meeting a trough. At these places, the waves weaken or
destroy each other. This interaction between waves is called
interference.
The interference happens in all kind of waves. The other enclosed
picture (originally one of our photographs, which does not scan
well) shows a pattern of water waves and the interference pattern.
A good way to see wave patterns is to use a ripple tank, which
your school or college should be able to demonstrate to you.
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