1. The problem statement, all variables and given/known data Consider a layer of oil assume n = 1.6 about 5 mm thick floating over a body of water n = 1.33. If a light ray is obliquely incident from air onto the oil surface find the range of incidence angles if any that results in total internal reflection at the oil water interface. 2. Relevant equations sin(theta_critical) = n2/n1 when n1 > n2 3. The attempt at a solution My first attempt would be to say that theta_crit = arcsin(1.33/1.6). However, that is incorrect. My professor said to use snells law and consider each interface. 1*sin(theta_incident) = 1.6 * sin(theta_2) n2*sin(theta_2) = n3 * sin(theta_3) 1.6*sin(theta2) = sin(theta_i)/1.33 = sin(theta_3) However, I can't seem to get anything useful out of this... The answer is that there is no incident angle that would found. I can see that for the air-oil interface 1.6/1 wouldn't have any critical angle, but why does the oil-water interface not have one as well?