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.