The Dutch scientist Huygens suggested a graphical method of predicting the future position of a wave-front. His principle is stated as follows.

In the following examples it is perhaps easiest to imagine waves on the surface of water (as observed in a ripple tank), but the results can be applied to any two (or three) dimensional waves.

First, a rather trivial example. Consider the set of plane (straight) waves shown below. The velocity of propagation of the waves is v. To apply the principle, we must pretend that we cannot guess where the wave-front will be, say, t seconds later!

First chose a point (any point), A, on the wave-front and draw an arc of radius vt. This is the distance the "secondary wavelets" will have moved in t seconds.

Now chose another point, B, at random, and repeat the process. The new wave-front is the tangent to the two curves…well what a surprise!

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