Refractive index & it's Significance

Refractive index:

Refractive index is the property of a medium which controls the velocity of light at that medium.

When light  moves from one medium to another medium then the refractive index of the next medium with respect to the first medium is given by the ratio of velocity of light in the first medium to the velocity of light in the next medium.

Let us consider a light beam moving from a medium ( say 1 ) to another medium ( say 2 ) as shown in figure.

Then refractive index of medium 2 with respect to medium 1 is given by

Significance of Refractive index :

CASE 1 :

Let us consider a beam of light, moving from medium 1 to medium 2, such that medium 1 is optically rarer medium (say air) and medium 2 is optically denser medium.

       

Then refractive index of medium 2 with respect to 1 is given by

Since, velocity of light in rarer medium is greater than velocity of light in denser medium. Hence,

                  V1  >  V2

     i.e.        μ21 > 1

Hence, we can conclude that whenever the refractive index of a medium is greater than 1, it defines that the light is coming from a rarer medium to an optically denser medium.

CASE 2:

Let us consider the light is moving from medium 2 to medium 1 that is from optically denser medium to rarer medium as shown in figure.

                     

             

Then the refractive index of medium 1 with respect to medium 2 is given by

            

Here,         V1 > V2

  i.e. ,         μ12 < 1

Hence, we can conclude that whenever the refractive index of a medium is less than 1, it defines that the medium is rarer with respect to the preceding medium( from where light is coming).

CASE 3:

For air ,       μ = 1