I have a example where a light goes into a triangular glass from air. I used the protractor to calculate the angle of incidence. I then used that angle and plotted in an equation to find the angle of refraction and then drew the refracted ray. So by now the light should exit the glass and enter air. I did the exact same thing as I did with the angle of incidence, except when i try to calculate the angle of refraction I get an error on my calculator. In this situation, what am i suppose to do? if im not clear enough please tell me.