The polarizing microscope is the essential tool for petrology.

It works on exactly this principle. There are two polarizing filters, and a very thin slice of rock is placed on a rotating stage. (There are three axis stages too, but it gets weird.) The minerals each have their own polarizing properties. Much can be discerned from this: mineral type, mineral composition, mineral growth zonation... I spent a lot of time learning the optics and staring down a tube until my eyes crossed. Now, there are much better microscope cameras that help with the eye strain.

Basically, that third middle lense is bending the light.