Reflection of Light by a Transparent Medium in Air

n (Fresnel’s formulae) If i is the angle of incidence, r the angle of refraction, n ₁ the index of refraction for air (nearly equal to unity), n ₂ index of refraction for a medium, then the ratio of the reflected light to the incident light is,

$$R = {1 \over 2}\left( {{{\sin ^2 (i - r)} \over {\sin ^2 (i + r)}} + {{\tan ^2 (i - r)} \over {\tan ^2 (i + r)}}} \right)$$

If i = 0 (normal incidence), and n ₁ = 1 (approximate for air),

$$R = \left( {{{n_2 - 1} \over {n_2 + 1}}} \right)^2 $$

(Weast RC (ed) Handbook of chemistry and physics, 52nd edn. CRC Press, Boca Raton, FL).