Snell's Law

(eq (/ n_1 n_2)
    (/ (sin Omega_2)
       (sin Omega_1)))

(Omega_1 "angle from boundry for first medium")
(Omega_2 "angle from boundry for second medium")
(n_1 "index of refaction of first medium")
(n_2 "index of refaction of second medium")


good example is the 22 degree halo: Ice (n=1.31), two refractions from 60
degree prism (hexagonal ice crystal)