Refractive Index of Medium 1 to Medium 2 :
Refractive index for ray of light going from medium 1 to medium 2 is equal to the reciprocal of the refractive index for ray of light going from medium 2 to medium 1.

This can be proved in following way:

Refreactive Indext of Light From Medium 1 to Medium 2 and Vice Versa Relation

Suppose in first case ray of light is travelling from air to glass. Here air is taken as medium 1 and glass is taken as medium 2.
Let velocity of ray of light in air is v1
and that of glass is v2
we also know 1μ2 = v1 / v2

In this case we can write
Refractive Index (R.I.) of medium 2 = v1 / v2 – – – – – – – – – 1

Refreactive Indext of Light From Medium 1 to Medium 2 and Vice Versa Relation second case

Again suppose in second case light is travelling from medium 2 to medium 1 . Refractive Index (R.I.) of medium 1
= v2 / v1 – – – – – – – – – 2
I am again writing the equation 1
Refractive Index (R.I.) of medium 2 = v1 / v2

or Refractive Index (R.I.) of medium 2 = 1 / (v2 / v1)

= 1 / 2μ 1

1μ2 = 1 / 2 μ1

This can also be written as :
airμglass = 1 / glass μ air

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