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Refraction of Light

The bending of the ray of light passing from one medium to the other medium is called refraction (Fig. 12).
  • The refraction of light takes place on going from one medium to another because the speed of light is different in the two media.
Fig. 12
  • Greater the difference in the speeds of light in the two media, greater will be the amount of refraction.
  • A medium in which the speed of light is more is known as optically rarer medium and a medium is which the speed of light is less, is known as optically denser medium.
  • When a ray of light goes from a rarer medium to a denser medium, it bends towards the normal (Fig. 13).
Fig. 13
  • When a ray of light goes from a denser medium to a rarer medium, it bends away from the normal (Fig. 14).
Fig. 14

Snell’s law

The ratio of sine of the angle of incidence to the angle of refraction (r) is a constant called refractive index.
i.e., 113197.png (a constant). For two media, Snell’s law can be written as 113203.png
μ1 × sin i = μ2 × sin r, i.e., μ sin θ = constant
Also in vector form: 113209.png

Refractive index

Refractive index of a medium is that characteristic which decides speed of light in it. It is a scalar, unitless, and dimensionless quantity.
Absolute refractive index When light travels from vacuum to any transparent medium, then refractive index of medium wrt vacuum is called its absolute refractive index, i.e., vacuumμmedium = c/v
Absolute refractive indices for glass, water, and diamond are, respectively, μg = 3/2 = 1.5, μw = 4/3 = 1.33, and μD = 12/5 = 2.4.
Relative refractive index When light travels from medium (1) to medium (2) then refractive index of medium (2) wrt medium (1) is called its relative refractive index, i.e., 113221.png (where v1 and v2 are the speed of light in medium 1 and 2, respectively).
  • When we say refractive index we mean absolute refractive index.
  • The minimum value of absolute refractive index is 1. For air it is very near to 1. (≅1.003)
  • Cauchy’s equation: 113240.png
    (λRed > λviolet so μRed < μviolet)
  • If a light ray travels from medium (1) to medium (2), then
Dependence of refractive index
  • Nature of the media of incidence and refraction
  • Color of light or wavelength of light
  • Temperature of the media: Refractive index decreases with the increase in temperature.

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