total internal reflection

[Nb]

The drawing shows an optical fiber that consists of a core made of flint glass (nflint = 1.667) surrounded by a cladding made of crown glass (ncrown = 1.523). A beam of light enters the fiber from air at an angle q 1 with respect to the normal. What is q 1 if the light strikes the core-cladding interface at the critical angle q c?http://edugen.wiley.com/edugen/courses/crs1000/art/images/c26/nw1183-n.gif

i know u have to use snell's law

sin theta = (n2/n1), but i don't know what to do from here.

The text book in which this problem is in gives no info on these kinds problems. Please help me [*(]

[Tom Mattson]

You have to use Snell's law twice. You have to do it once to find the critical angle (considering the cladding-core interface) and then you have to do it again to find θ1 (considering the core-air interface).