In the above formula for , if we put (Snell's law) and multiply the numerator and denominator by , we obtainFresnel, 1866, p.773.Hecht, 2002, p.115, eq.(4.42).
In the above formula for , if we put (Snell's law) and multiply the numerator and denominator by  we obtainFresnel, 1866, p.773.Hecht, 2002 p115, eq.(4.42).