As what you see. a perfect circle by circle brush. Left side works fine just like what written on the stupid high school physics textbook But the right side was a big discover in physics: reflect angle no more than 90 degrees
there is a circle drawing problem on windows,the right side is missing some random pixels.
EDIT: hmm it doesn't seem that is the problem, as i get this too, i will look into it. I will get correct bounces for a few photons, then the rest will not go past that 90 degree point