It is possible that mass
density of an optically denser medium may
be less than that of an optically rarer
medium (optical density is the ratio of the
speed of light in two media) For example,
turpentine and water Mass density of
turpentine is less than that of water but
its optical density is higher 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