The fact that sunlight is absorbed by water is well known to any diver who has dived to a depth of 100 ft. It is also true that the intensity of light falls exponentially with depth. Suppose that at a depth of 25 ft the water absorbs 15% of the light that strikes the surface.
At what depth would the light at noon be as bright as a full moon, which is one three-hundred-thousandth as bright as the noonday sun?