Snell's law is used for refraction of light
it is used to find the relation between refractive index of two medium and angle of incidence and angle of refraction.
As per this formula
[tex]n_1 sin\theta_i = n_2 sin\theta_r[/tex]
here [tex]n_1[/tex] = refractive index of first medium
[tex]n_2[/tex] = refractive index of second medium where light go after refraction
[tex]\theta_i[/tex] = angle of incidence
[tex]\theta_r[/tex] = angle of refraction
