What is the equation for Snell's law?

where
##n_1## is the index of refraction of the first substance that light travels through
##theta_1## is the angle of incidence
##n_2## is the index of refraction of the second substance that light travels through
##theta_2## is the angle of refraction
Then you need to isolate your unknown.
Example:
Light in air (##n_1## = 1.0003) is incident upon a block of crown glass (##n_2## = 1.52) at an angle of ##35.0^o##. What is the angle of refraction?
##n_1## sin ##theta_1## = ##n_2## sin ##theta_2##
(Isolate sin ##theta_2## by dividing both sides by ##n_2##)
##n_1## sin ##theta_1## / ##n_2## = sin ##theta_2##
(1.0003) (sin 35.0) / 1.52 = sin ##theta_2##
0.5737 / 1.52 = sin ##theta_2##
0.3775 = sin ##theta_2##
(To find ##theta_2## you need to find the inverse sine of this value. On your calculator press the 2nd function key then the sine key.)
##22.2^o## = ##theta_2##