When light rays angle down toward the surface of still water, some of the rays bounce off the surface (reflection) and some travel down into it (refraction). Thanks to refracted light, we’re able to see the bottom, and the water looks transparent.
This painting by the Russian landscapist Zhukovsky shows both reflections and transparency. (The image is from Agni Art, a good source for inexpensive prints of Russian paintings.)
Above, the water is almost entirely transparent, with just a few slithery slashes of blue sky reflections to suggest the moving stream. (From the John Singer Sargent Virtual Gallery, a website that catalogs all his works.)
In three previous posts: (Part 1, Part 2, Part 3) we looked at water reflections, but this time, let’s also consider transparency.
The relative amounts of reflected and refracted light depends on the angle that the rays touch the water surface. For this reason, when you look steeply down into water, it looks more transparent, and when you look straight out across the water, all you see are reflections.
In the lower right of this study, you can see the streambed because you’re looking steeply downward, but higher up in the scene, the blue reflections of the sky take over.
Same thing in the study below. There’s mostly sky reflection at (1) and there’s more transparency at (2). In the area marked (3), the tones of the riverbottom are darker because the reflected skylight is interrupted by the mass of the rock. Polarized sunglasses will also selectively remove some of the glare of reflected skylight, allowing you to see more of the transparency (or refracted) rays.
In (4) you can see the edge of the last high tide. The tide was coming in as I painted this, covering the rocks one by one, and darkening them as it did so. Because blue light is scattered away and subtracted from the light illuminating subsurface rocks, they look darker and warmer than the rocks above the surface.