Why does P polarized light dip in reflectance around 55 deg and why only that vs S polarized or unpolarized light?
Hi great question. The dip to zero at ~55 degrees is known as Brewster's angle. I wasn't sure about this one, but here are Professor Ren and Bob's responses summarized: the plot depicts how light interacts w/ surfaces made of dielectric material. The derivation of the interaction is out of scope, but here are some slides that describe the different polarization behaviors https://www.brown.edu/research/labs/mittleman/sites/brown.edu.research.labs.mittleman/files/uploads/lecture13_0.pdf).
I found the Wikipedia article on Brewster's angle helpful as it gives some visual examples of how this affects images filtered with different polarizer filters: https://en.wikipedia.org/wiki/Brewster%27s_angle.