2  HUYGENS PRINCIPLE
We would first define a wavefront: when we drop a small stone on a
calm pool of water, waves spread out from the point of impact Every
point on the surface starts oscillating with time At any instant, a
photograph of the surface would show circular rings on which the
disturbance is maximum Clearly, all points on such a circle are
oscillating in phase because they are at the same distance from the
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