Waves Unit- Angle of Incidence

Homework Statement

http://imgur.com/LDx7I

Homework Equations

http://en.wikipedia.org/wiki/Snell's_law

The Attempt at a Solution

I'm not really sure on the approach to take for this question. Since it proves 2 angles, Snell's Law could prove to be beneficial.

Thanks for the help!

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First , use the snell's law on the bottom surface to connect the critical angle to the refractive index of the block. For the vertical surface, the angle of transmission is $90-\theta_c$ using basic geometry. then use snell's law again at the vertical surface. at the vertical surface angle of incidence is $\theta_1$