I was standing outside in very light drizzle, sun behind me. I saw a rainbow. I know why they occur but…
I was wearing polarized sunglasses. As an experiment, I turned my sunglasses through 90 degrees. The rainbow got brighter.
I partially expected this, since rainbows are very directional.
What I did not expect is that my naked eye (without sunglasses) saw a dimmer rainbow than the eye looking through the polarized dark glass. I had expected it to be about the same.
More than that, through the sunglasses the colours were more vibrant.
What are the reasons for this? I didn't think refracted light could be that polarized.
Best Answer
The rainbow did not become brighter through the polarized sunglasses (PS). Rather, the PS enhanced the contrast between the rainbow and the background light of the sky: The PS decreased the brightness of the sky, while the effect on the rainbow, if any, was much smaller. While the eyes have adjusted to the absolute level of brightness, the relative brightness of the rainbow (i.e., contrast) became higher.
The reason why the colors appear more vibrant through the PS is the same: higher contrast.
And, just in case, the reason why PS enhance the contrast is described in detail in the section “Sky polarization and photography” of the Wikipedia article on polarization.