Refraction of light : Law of Refraction and Refractive index

29/06/2020 Vinod 0 Comments

Refraction of light :-

when a beam of light encounters another transparent medium, a part of light gets reflected back into first medium while the rest enters the other, The direction of propagation of incident ray of light that enters the other medium changes at the interface of the two media. This phenomenon is called refraction of light.

Law of Refraction :-

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Fig. 1. Ray diagram for refraction of light.

Snell experimentally obtained the following laws of refraction.

(a) The incident ray, the refracted ray and the normal to the interface at the point of incidence, all lie in the same plane.

(b) The ratio of the sine of the angle of incidence (i) to the sine of angle of refraction (r) is a constant (μ).

I.e sin i/sin r = constant

Or sin i/sin r = μ

Where μ is a constant called refractive index of the medium.

The equation is known as Snell’s law of refraction.

  • If μ > 1, r < i that means the refracted ray bends towards the normal in such a case medium 2 is said to be optically denser than medium 1
  • If μ < 1, r > i that means the refracted ray bends away from the normal. In such a case incident ray in a denser medium refracts into a rarer medium.

Not : – μ is depends on the wave length of light but is independent on the angle of incidence.

Refractive index :-

The ratio of the velocity of light in a vaccum to its velocity in a specified medium is called refractive index of material medium.

i. e μ = c/v

Where c is the velocity of light in vaccum, v is velocity of light

in the medium and μ is called absolute refractive index of the medium.

Relative Refractive index :-

the ratio of absolute refractive index of two media is called relative refractive index.

If μ₁ is the refractive index of medium 1 and μ₂ is the refractive index of medium 2 then the relative refractive index of medium 2 with respect to the medium 1 is

μ₂₁ = μ₂/μ₁

Thus, for medium 1 with respect to the medium 2 is

μ₁₂ = μ₁/μ₂

  • If velocity of light in the medium 1 and 2 is v₁ and v₂ respevtivelly, then

μ₂₁ = μ₂/μ₁= c/v₂ / c/v₁ = v₁/v₂

μ₁₂ = μ₁/μ₂ = c/v₁ / c/v₂ = v₂/v₁

Hence μ₂₁ × μ₁₂ = v₁/v₂ × V₂/v₁ = 1

i. e μ₂ = 1/μ

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