Refraction

The ability of a medium to refract light is also expressed in terms of its optical density.
Optical density has a definite connotation. It is not the same as mass density. We have
been using the terms ‘rarer medium’ and ‘denser medium’ in this Chapter. It actually
means ‘optically rarer medium’ and ‘optically denser medium’, respectively. When can
we say that a medium is optically denser than the other? In comparing two media, the
one with the larger refractive index is optically denser medium than the other. The other
medium of lower refractive index is optically rarer. The speed of light is higher in a rarer
medium than a denser medium. Thus, a ray of light travelling from a rarer medium to a
denser medium slows down and bends towards the normal. When it travels from a
denser medium to a rarer medium, it speeds up and bends away from the normal.