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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?

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    $\begingroup$ It's not so much a shadow as it is a bending of light away from a certain point. $\endgroup$ Commented Mar 19, 2015 at 18:32
  • $\begingroup$ yeah ,but is the bending effect of superposition of light by waves? $\endgroup$ Commented Mar 19, 2015 at 18:37

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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.

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    $\begingroup$ 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... $\endgroup$
    – Jon Custer
    Commented Mar 19, 2015 at 19:46
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The dark part of the shadow is the umbra, and the part that is a little lighter is the penumbra. They can be experienced on Earth, but more readily in space, such as during a solar eclipse, when the Moon moves in front of the Sun and leaves a shadow on Earth.

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  • $\begingroup$ The ripple image is due to refraction (& maybe reflection), it's not actually a shadow. $\endgroup$
    – PM 2Ring
    Commented Mar 15, 2020 at 4:03
  • $\begingroup$ I agree with @PM2Ring $\endgroup$ Commented Apr 25, 2020 at 19:23

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