1. The problem statement, all variables and given/known data A light ray passes downward into a block of transparent plastic with an angle of incidence of θ1 = 77.7°. If total internal reflection is to occur when the light strikes the left edge of the block at interface B, what is the index of refraction of the plastic? 2. Relevant equations Snell's Law - n1sin(θ1)=n2sin(θ2) sin(θcrit)=n2/n1 3. The attempt at a solution I thought that I could use the law of refraction at interface A and the equation for the critical value at interface B (since total internal refraction occurs there), and use those two equations to solve for n2. I've done all manners of substitutions, but nothing that yields a reasonable answer. I would really appreciate a nudge in the right direction. Thanks in advance!