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I was filling the bathtub the other day with water and noticed something. When calm, the light illuminates everything and I can see the sides and bottom of the white tub without shadows. Then, when I add a droplet of water its ripples (either its peaks or valleys of the wave) cast concentric rings of shadow which travel about 2 feet per 3 seconds (0.2 m/s) outward from epicenter.

I'm wondering what exactly creates these shadows. That distance is too large for light waves destructively interfering.

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  • $\begingroup$ Can you do this experiment with distilled water? If there are those dark ripples there then we can rule out impurities. I will conduct some simple experiments and address this question tomorrow. $\endgroup$ – Sad_lab_rat Apr 15 '17 at 8:42
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What you see in a lensing effect by the ripples on the surface of the water. There will be regions on the surface where the water surface (like a crest) acts as a converging lens and regions where the water surface acts like a diverging lens (troughs).
Thus the illumination of your screen will vary.
You have made yourself a ripple tank.

Here is a ray diagram to show the general idea of what is happening.

enter image description here

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