A ray of light strikes a square glass block at an angle of 45 degrees and enters the glass. If this block is surrounded by air (n=1), what is the minimum value of the refractive index of the glass if the total internal reflection occurs at point P. (see attached diagram) I'm really lost on this one. I know to take Snell's law at the surface AD. And the condition for the total internal reflection at the surface AB. I know n1sin(45) = n2sin(theta) from Snell's law, but how do I get the value for the unknow angle (theta)? I know that we can determine that the angle at P is sin(90-theta) which = cos (theta) And I know that sin2(theta) + cos2(theta) = 1. But that's as far as I can get. Any help would be appreciated. I've attached a diagram for clarification. Thanks.