If a light beam from a medium (e.g. air) with refractive index $$n_1$$ enters a medium (e.g. water) with refractive index $$n_2$$ at angle $$\alpha_1$$ (between the vertical and incident beam), the refractive angle $$\alpha_2$$ (between the vertical and outgoing beam) is determined by the Snell's law:$$n_1 \, \sin(\alpha_1) ~=~ n_2 \, \sin(\alpha_2)$$