Refractive Index of a material is defined as ratio of:

CONCEPT:

• Refractive index: The ratio of the speed of light in a vacuum to speed of light in a medium is called the refractive index of that medium. It is also called an absolute refractive index.

\({\rm{Refractive\;index\;}}\left( \mu \right) = {\rm{\;\;}}\frac{{Speed\;of\;light\;in\;vaccum\left( C \right)}}{{Speed\;of\;light\;in\;a\;medium\left( v \right)}}\)

• Relative refractive index: When a light ray is going from one medium (other than vacuum) to another medium then the value of the refractive index is said to be a relative refractive index.

\({\rm{Refractive\;index\;of\;medium\;}}1{\rm{\;with\;respect\;to\;medium\;}}2{(n_{21})} = {\rm{\;}}\frac{{\;refractive\;index\;of\;medium\;1 {(n_1)}}}{{\;Refractive\;index\;of\;medium\;2{(n_2)}}}\)

EXPLANATION:

• The speed of light in a vacuum is maximum (\(3 \times {10^8}\) m/s) and the speed of light in air is almost same as in a vacuum. The speed of light in the liquid material is less than that in air and the speed of light in a solid is less than that in liquid.
• Thus the refractive index of solid > Refractive index of liquid > Refractive index of air.
• The Refractive index of material with respect to air will be equal to the ratio of the refractive index of the material to the refractive index of air.

\(Refractive\;index\;of\;material\;with\;respect\;to\;air = \frac{{Refractive\;index\;of\;material}}{{Refractive\;index\;of\;air}}\)

• As the refractive index of any material is more than that of air always. So the refractive index of material with respect to air will be always greater than 1.
•  Thus Refractive index of material with respect to air > 1

Table: