The Question: A 45-45-90 prism is immersed in water. What is the minimum index or fefraction the prism must have if it is to reflect totaly a ray incident normally on one of its faces. Now, we have found this question a bit hard to understand. We assume that light is hitting one of the sides at 90 degrees (normally) and so will pass into the prism without refraction. Thus, it hits the other side of the prism at an angle of incidence of 45 degrees. This is where we get stuck, we are assuming that it is totaly internaly reflecting and by using snells law nSin(45) = Sin(90) we find that n, the refractive index of the prism is 1.88 This complies with the fact that to be totaly internaly reflected the light must travel from a high refractive index (1.88) to a low refractive index (1 for air) If we are on the right track, please let us know!