Is sunlight polarised?

I just got a new pair of polarised sunglasses, and I noticed that if I turn them, the colour of the blue sky outside gets darker or lighter every 90 degrees.

AFAIK the sunlight is not polorized. It is once it reflects off of a non metallic/shiney surface. Then it is partially polarized. Turn your glasses 90 degrees when looking at the reflection of the sunlight (or any light for that matter) off of something dull (but reflective) like a waxed surface. Anyways since the blue sky is actually reflected sunlight, and between that and passing through the atmosphere I’d imagine that somewhere it manages to polorize it self.

Direct sunlight is not polarized. Well, if you use high-resolution telescopes and measure polarization around sunspots, etc., you can do a lot of interesting science, but the polarization of the overall sunlight is negligible.

The blue light from the sky, on the other hand, is light scattered by the air - mostly oxygen molecules, IIRC. This is Rayleigh scattering, and the scattered light is very strongly polarized.

Photographers know this well. Using a polarized filter you can adjust the brightness of the sky at will. Without a filter, the daytime sky is too bright and washed out. With a polarized filter set to reject the scattered light, you get a nice deep blue sky, suitable for postcards of tropical islands.

Wonderful explanation. It, of course, was the sky not the sun I was looking at. Thanks.