Wave Interference:

Think about waves on a pond.  If you drop a stone into a 
body of water what do you see?

The stone acts as a source of water waves that travel 
outwards from a point, forming ever larger circles.  You 
will see wave peaks and troughs, corresponding to areas 
where the water is higher or lower.

What happens now if you take a stone in each hand, and 
drop them both into the pond?

This time you will initially see the same pattern of 
outgoing waves from each stone, but look carefully at 
what is happening when they start to overlap.  If you 
concentrate on one particular wave front, you will see 
that instead of remaining a peak along its whole length, 
there are regions where it now looks flat.  What you are 
seeing is an interference pattern.  In areas where two 
peaks overlap, the resulting peak is twice as high as the 
original peaks, and in areas where a peak and trough 
overlap, the result is a region of level water. 

(Sketch of two overlapping circular wavefront patterns 
here…)







(Want sketches of constructive and destructive 
interference here…)









A similar thing happens with sound waves.  If we take two 
sources of (identical) sound waves, for example two loud 
speakers producing the same sound, we will find as we 
walk around them that there are areas where the sound 
almost disappears and areas where it is very loud.  We 
can learn something about the wavelength of the sound 
waves by seeing how far apart these maxima and minima 
are.