Deriving Brewster's Law

Actually, I have completely understood Brewster’s Law, but I have not understood the assumption in the beginning. Why is it that the reflected light is completely polarised and the refracted ray only partially polarised when they are inclined at an angle of pi over two radians? Is this an experimentally determined fact, or is there some cause to it?
This is an image that might help you understand my question:
https://i.stack.imgur.com/3LU1P.png