What causes the ligher and darker areas, or shadows in a white cup of hot water when you ( gently ) stir it? I’m guessing it has something to do with tiny variations in the temperature of the water, and that either hot or cold allows less light to pass, perhaps?
Shadows of ripples happening on the surface as you stir (or the refractive equivalent of shadows), would be my guess.
That’s not to say that other factors, including temperature, don’t alter the light transmission properties of water, but if you’re stirring it, you’re disturbing the surface and that’s going to affect the paths of light rays on their way through.
I thought the same thing at first. But even the slightest of a stir that does not effect the surface will animate the “shadows”.
I think regions of the water at different temperatures (which will exist before it is well stirred) will have different refractive indexes (indices?), and so bend the light through different angles.
I just did a little test by filling one cup with hot water. The moving shadows are somewhat visible even without stirring I now notice. But when I slowly add a spoon of cold water, then I can see clear ripples in the water as the cold water moves through the hot water.
Come to think of it, this looks almost the same as heatripples in the air above, say, black asfalt on a sunny hot day.