If n 21 is the refractive index of medium 2 with
respect to medium 1 and n12 the refractive index
of medium 1 with respect to medium 2, then it
should be clear that
12
21
1
n
=n
(9 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)