and the angle of refraction 
are related by Snell's law:
from: one medium into another
states: that
(i) the incident ray, the refracted ray and the normal to the boundary all lie in one plane, and
(ii) the angle of incidence and the angle of refraction are related by Snell's law:
where 
and 
are the refractive indices of the two media, respectively. (The refractive indices normally depend somewhat on the frequency of the light giving rise to dispersion.) [P6.1, P6.2]
Copyright 1997, The Open University