What do you mean "show"?
That is Snell's law
n1 sin theta 1 = n2 sin theta 2
Do you want to prove it by using speed of the wave in the two media or something?
show that nsini =constant
where n is refractive index and i is angle of incidence
3 answers
If so, look at the diagram of the two parallel rays here:
https://math.stackexchange.com/questions/153775/simple-proof-for-snells-law-of-refraction
https://math.stackexchange.com/questions/153775/simple-proof-for-snells-law-of-refraction
https://math.stackexchange.com/a/2496002