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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1$\begingroup$ It's not so much a shadow as it is a bending of light away from a certain point. $\endgroup$– NeutronStarMar 19, 2015 at 18:32
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$\begingroup$ yeah ,but is the bending effect of superposition of light by waves? $\endgroup$– Shafayet RahatMar 19, 2015 at 18:37
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2 Answers
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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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1$\begingroup$ 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... $\endgroup$ 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 2RingMar 15, 2020 at 4:03
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