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Refractive index

 

What is refractive index?

Refractive index:

In general refractive index of any material or medium explains about its optical density.

The medium which has greater value of refractive index is considered as denser medium and the medium which has lesser value of refractive index is considered as rarer medium.

So to decide less refractive index or more refractive index we need to depend on a reference.

Here we compare the refractive indices of all other media with the refractive index of air to decide whether they are denser or rarer, Because refractive index of air is our reference.

NOTE: Refractive indices of air and vacuum are taken same by approximation even though they are not exactly same.

Refractive index of air is always considered as 1.

How many types of refractive indices are there?

i)                   Absolute refractive index

Absolute refractive index is the ratio between speed of light in vacuum and speed of light in a particular medium. 

        Refractive index can be denoted with a symbol ‘n’ or ‘µ’

Therefore absolute refractive index   µ=c/v

Here ‘c’ is speed of light in vacuum and ‘v’ is speed of light in a particular medium. So this ratio will give the value of refractive index of a particular medium (this value is absolute).

What are the units to measure refractive index?

Refractive index doesn’t have any unit or dimensional formula as it is the ratio of same physical quantities (speed), hence both will get cancelled.

µ∞1/v
 
In the equation µ=c/v, we know ‘c’ is always constant. Therefore refractive index of a medium is inversely proportional to speed of light in that medium.



The below table is an example of absolute refractive indices of different media.


Refractive index table 1

Relative refractive index:

Here we have a word ‘relative’ which is explaining that we are comparing the refractive indices of two media.

For example let us consider two optical media with different refractive indices.

Let refractive index of first medium be ‘µ1’

And let refractive index of second medium be ‘µ2’

Now if you want to compare refractive index of first medium with respect to second medium then we have to take the ratio of refractive index of first medium and refractive index of second medium.

µ1/µ2   or simply id can be denoted as µ12   which gives the meaning, refractive index of first medium with respect to refractive index of second medium.

Similarly µ21 means refractive index of second medium with respect to refractive index of first medium.

As we know that, µ∞1/v

Therefore   µ1/µ2=v2/v1

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