Brewster angle

In wave optics, the Brewster angle ${\theta }_p$ is the angle of incidence at which $r_{TM}=0$.

$${\theta }_p=\arctan \left(\frac{n_t}{n_i}\right)$$

This has some physical significance. The result of $r_{TM}=0$ is that all TM-polarised light is completely transmitted with no reflection. This occurs when the reflected and refracted rays are perpendicular to each other.