Why are rainbows brighter through polarized glass? 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.
 A: A bit too long for a comment:
If we consider a short "arc" of the rainbow, all the light contributing to the rainbow (as opposed to background light) would probably tend to be polarized in the same direction.
This is because the "rainbow light" was first refracted when it entered a water droplet through the "front" hemisphere of the droplet surface, then reflected on the "back" of the droplet surface, and finally refracted again when leaving the droplet again through the "front" surface. The geometry will be nearly the same for all the droplets within this short arc of the rainbow.
This refraction-reflection-refraction sequence will favor some polarization directions, as I said.
I suspect this is why, when you turn your sunglasses correctly so they match this polarization direction, the rainbow will be sharper: The background light from the daylight sky (which tends to outshine the rainbow) will be dimmed much more than the rainbow itself.
Maybe the daylight from the sky almost opposite the sun is polarized as well (whether or not there is any rainbow). Did you see this effect near the "top point" of the rainbow where the rainbow arc is close to "horizontal", or closer to one or both "legs" of the rainbow where the arc is more "vertical"?

After googling, I found a YouTube video showing how great this effect is.
A: 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.
