What is the Correct Equation for Deviation by a Thin Prism?

[shr]

Homework Statement

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?

Homework Equations

A scan of the derivation, from our textbook:

The Attempt at a Solution

I googled, but did not find anything interesting.

 

[rl.bhat]

General expression for Snell's law is
n1*sini1 = n2*sini2 where 1 suffices indicates the incident medium and suffices 2 indicates refracted medium.
Now apply the law to the face AC of the thin prism ABC.

 

[shr]

General expression for Snell's law is
n1*sini1 = n2*sini2 where 1 suffices indicates the incident medium and suffices 2 indicates refracted medium.
Now apply the law to the face AC of the thin prism ABC.

RL, I figured it out myself after asking the problem:)

The root of the problem arose from our textbook. They had, at first defined refractive index as "sin i/sin r" but did not mention that it only holds good in the case where a light ray falls from air(n = 1) into another medium. That's where I fell :)

Thanks for your reply.