11)
It also follows that if n 32 is the refractive index
of medium 3 with respect to medium 2 then  n 32 =
n 31 ×  n 12, where  n 31 is the refractive index  of
medium 3 with respect to medium 1 Some elementary results based on the laws of
refraction follow immediately For a rectangular
slab, refraction takes place at two interfaces (air-
glass and glass-air) It is easily seen from Fig