Imagine you're at a lake at twilight. The water is still, reflective. You see a dark spot on the water that isn't in the sky.
You're standing right at the water's edge, with the sun's placement on your left or right, not in front or behind you.
Look at the surface of the water about 4 feet in front of you. Examine the reflection of the blue, twilight sky.
What you're seeing is a result of polarized ight. To see it on a lake, you need patience, luck, a calm lake, and a clear twilight sky.
The sun has to be on the horizon, just setting. The water has to be like glass. The sky has to be clear blue; a few small clouds are okay, but no haze.
Polarization of light is a quality of light the human eye is not sensitive to like some fish and other animals are.
Like A Vibration
We can think of light as a vibration, something like a vibration traveling along a stretched rope. Light that vibrates in some clearly defined direction is said to be polarized.
The blue light of the sky is polarized. When the sun is low to your left or right, as it is in this experiment, the blue light of the sky right in front of you is polarized vertically; the vibrations are up and down.
Vertical Vs Horizontal
But the lake doesn't reflect that light very well. Horizontal surfaces in general don't reflect vertically polarized light very well. You see a dark patch in the lake 4 feet in front of you because, at that angle, the lake doesn't get much light it can reflect.
The blue sky supplies vertically polarized light; the lake, being horizontal, can't reflect it and that's why there's a mysterious dark patch in the reflection of the sky.
Sources And Further Reading:
- G.P. Konnen. Polarized Light In Nature. Translated by G.A. Beerling. Cambridge University Press. 1985.
- M. Minneart. The Nature of Light & Colour in the Open Air. Translated by H.M. Kremer-Priest, revised translation by K.E. Brian Jay. Dover Publications Inc. 1954.