total internal reflection - snell's law?
 

light from air strikes a translucent plastic material. if the light strikes at an angle of 30 degrees to the material and bends by 15 degrees, what is the index of refraction of the plastic.
i obtained an answer of n=1.932, but the book gave an answer of n=1.22, what am i doing wrong? please help and explain in detail.
thanks in advance!