Refractive Index

Light travels at a speed of $3times10^8$ in vacuum. Light travels at different speeds in different materials because as the light enters any material the atoms of the material absorb, scatter and remit light. Therefore, when it travels from one medium to another, the speed either increases or decreases. The amount by which its speed changes determines the amount by which it will change its direction.

The speed of light in vaccum is $3times10^8$ . The speed of light in a transparent medium such as glass, water or clear plastic is less than this. To calculate the amount by which the ray will deviates from its path we use a physical quantity known as refractive index. It is the ratio of the speed of light in vacuum to the speed light in that medium. As the refractive index is a ratio of two same quantities that is speed it does not have any units.

$Refractive ;Index; eta =frac{Speed; of; light ;in ;Vaccum}{Speed ;of; light; in; that; medium}= frac{c}{v}$

where c = the speed of light in vacuum and v = the speed of light in the medium.

In all material media the speed of light is less than c. Hence, the refractive index of a material medium is greater than 1. The speed of light in air at atmospheric pressure is very close to that in vacuum, and we generally take the refractive index of air as 1.

The refractive index of water is about 1.33, and that of ordinary glass and diamond are about 1.50 and 2.42 respectively.

Larger the refractive index of a medium, greater is the bending of light when it enters the medium from air obliquely. Thus light bends more when it enters a diamond than when it enters an imitation jewel made of glass.

Illustration:: A ray of light enters a diamond from air. If the refractive index of diamond is 2.42, by what per cent does the speed of light reduce on entering the diamond?

Answer: $eta :=:frac{c}{v}$

$v;=;frac{c}{eta }:=;frac{c}{2.42}:=;0.41c$

The speed of light in diamond is therefore 41% of its speed in air. In other words, in diamond, the speed reduces by 59%.

Sample Questions

(More Questions for each concept available in Login)

Question : 1

Which of the following is a unit of refractive Index?
A. m/s	B. degrees	C. Watt	D. Dimensionless
Right Option : D
View Explanation

Question : 2

Refraction is the change in the direction of light due to change in ____________of light
A. intensity		B. frequency
C. speed		D. both (b) and (c)
Right Option : C
View Explanation

Question : 3

If the velocity of light in vacuum is $3 X 10^{8} m/s$ , the refractive index of water is when the velocity of light in water is $frac{9}{4} X 10^{8} m/s$
A. $frac{5}{3}$		B. $frac{4}{3}$
C. $frac{3}{4}$		D. $frac{3}{5}$
Right Option : B
View Explanation