Basic angles of refraction and incidence help needed (help still needed) I feel silly for even asking this, but here goes. The triangle represents a glass prism. The refractive index between air and the prism is 1.55. The angle of incidence (angle 'p' on my diagram) is 45 degrees, and by using the equation sin r = sin i / 1μ2, I've worked out angle 'q' to be roughly 21 degrees. Knowing this, how could I work out angle 'r' on my diagram? I've tried visualising the two normal lines as two equals sides of an isoceles triangle, but I don't know whether that's just an illusion caused by the way I've drawn the diagram. I've also just assumed the angle 'r' is 90 degrees - 21 degrees, but I can't prove whether this is true or not. So, could anyone assist me with working out angle 'r' on my diagram? A few hints or pointers?