The question states: "For the right-angle prism shown, determine the maximum angle of incidence theta for the ray to be totally internally reflected at the hypotenuse face. Take the refractive index of the glass to be 1.52. First show that a ray with theta = 0 will be totally internally reflected. (Notice that this effect places a limit on the convergence or divergence angle of an incident beam, if the prism is to be used as a reflector.)" Here's what the picture basically looks like: http://s23.postimg.org/wibto5u4r/tri.png (it's a 45-45-90 triangle) Notice that it's asking for the maximum angle, not the minimum (ie the critical angle). For reference, I found the critical angle with Snell's Law, which is 41.1 degrees. A classmate told me that I need to find some complementary angles and make some expressions in terms of the incoming and refractive angles, but I am absolutely terrible at geometry, so any help is appreciated.