1. The problem statement, all variables and given/known data Ligh is incident normally upon one face of a prism of glass with an index of refraction, n. The light totally reflected as shown in the picture*. What is the minimum value n can have? *Picture shows a Porro prism (45-45-90 prism) 2. Relevant equations n sin(a) = n2 sin (b) setting b = 90 will solve for the critical angle 3. The attempt at a solution I guess it has something to do with critical angles. I just don't know how to use the information given. If its incident normally, then does sin(a) = 0? And if the light is not totally internally reflected then, the index of refraction for the prism must be greater than the index of refraction for air (1). so n(prism) > 1.