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When I go to the pool, I often see light patterns on the wall as shown in these two videos:

  1. Light patterns on wall at pool
  2. More light patters on wall at pool

I am trying to understand the physics behind these patterns.

My first question is: Are these patterns caused by light interference? That is, are the dark spots caused by destructive interference, and are the white parts caused by constructive interference? If this is some other phenomena, please explain.

If these are in fact light interference patterns, how does the light coming down from the window (this is in the morning, around 9 AM, and the sunlight is coming through a window at an angle) bounce off the water waves (I only see this happen when people are in the pool creating waves), and create these patterns. I would expect that the light of different frequencies bouncing off various parts of the water waves would just create a washed out white image on the wall. I don't really understand how all these different light frequencies, bouncing at different angles off of different heights of water waves could form interference patterns. Please explain.

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  • $\begingroup$ Why the down vote? What is unclear or what other research should I have done? $\endgroup$ – Greg Thatcher Oct 16 '16 at 17:51
  • $\begingroup$ This is a good question, because its answer is either trivial, or extremely hard. The trivial answer is the one given by Kevin Reid (I am not dismissing the answer by calling it trivial: it,s a good answer). The hard answer is to understand why you see the patterns you see rather than other patterns, and that is depends on why the waves on the pool have the characteristics they do, which is not simple to understand (and is probably at least in part an open question). $\endgroup$ – tfb Oct 16 '16 at 19:28
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These are not interference patterns; they are caustics.

The white light is reflecting off the surface of the water, and since the surface is irregular and changing, the reflection is jumbled and changing.

If the water surface were flat, it would reflect like a piece of glass, and you would see a flat white reflection on the wall. Instead, the surface is concave in many places (the troughs of waves) and those concavities focus the light down to bright spots or lines.

Physically, this is the same kind of thing as a bright spot made by reflecting sunlight off a curved mirror — it is just not a man-made carefully shaped mirror, but random ripples in the water surface, so the result is a mess.

The pattern is indirectly caused by wave behavior in that water waves propagate across the surface of the water, but there is nothing particularly about light waves here (other than that light can reflect off a surface at all).

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  • $\begingroup$ It could perhaps be emphasized that in caustics, it's not just a 1-to-1 mapping, where the even field of illumination that would result from reflection off a flat surface is instead streched & shrunk into more focused spots. (You could get that with very, very mild ripples & close wall, but the results wouldn't be caustics.) Rather, in many spots, the varied reflection angles cause light bouncing off non-adjacent areas on the water surface to get superimposed on the wall, with those overlaps brighter, and especially bright areas occur at the folds in the mapping that reverse the direction. $\endgroup$ – Jacob C. Oct 5 '18 at 21:42

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