Total Internal Reflection
Key Questions

Snell's Law can be used to calculate the critical angle for total internal reflection.
#n_1 sin theta_1 = n_2 sin theta_2# where
#n_1# is the index of refraction of the incident substance
#theta_1# is the unknown critical angle
#n_2# is the index of refraction of the refracting substance
#theta_2# is the angle of refraction (#theta_2 = 90^o# in a critical angle question)Isolate
#theta_1# by dividing both sides by#n_1# Then once you have solved for
#sin theta_1# , remember to take the inverse sine to find the angle of incidence. 
This question was expertly answered by Jeff C. at http://socratic.org/questions/howdoestotalinternalreflectionoflightwork