Suppose a stone is thrown into a pool of crystal clear water and its a bright sunshiny morning. You can observe a shadow of the wave in the bottom of the pool. Why does this happen? Is it due to superposition of light or some other thing?

  • It's not so much a shadow as it is a bending of light away from a certain point. – Joshua Mar 19 '15 at 18:32
  • yeah ,but is the bending effect of superposition of light by waves? – Shafayet Rahat Mar 19 '15 at 18:37
up vote 5 down vote accepted

You can actually think of the ripple as a travelling lens.

If you take a radial cross section through the ripple, it'll have a curved profile. Now just like a magnifying glass causes a bright spot in the middle of where you focus incoming light, it also causes a darker region around it.

This is what you're seeing on the bottom of the pool: The band of focussed (or unfocussed, depending on depth) light caused by the lens-like nature of the ripple wave.

  • I was also wondering about the contribution of varying reflectivity from the changing angle of incidence at the water surface. Hmmmmm - maybe I should calculate that some time... – Jon Custer Mar 19 '15 at 19:46

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