1. The problem statement, all variables and given/known data This really is not a Homework question, but I guess it fits in here. So, according to snell's law the ratio of sine of angle of incidence to the sine of angle of refraction is a constant called refractive index. But, in our textbook while deriving an expression for "Deviation by a thin prism", Snell's law seems to be used incorrectly. They have written "n = sin (refracted)/sin (incident)", which I do not understand, until now. Probably I'm wrong because I'm just a learner. Do you guys have any idea? 2. Relevant equations A scan of the derivation, from our textbook: 3. The attempt at a solution I googled, but did not find anything interesting.