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Total Internal Reflection

When a ray of light goes from denser to rarer medium it bends away from the normal and as the angle of incidence in denser medium increases, the angle of refraction in rarer medium also increases and at a certain angle, angle of refraction becomes 90°. This angle of incidence is called critical angle (C).
When angle of incidence exceeds the critical angle, then light ray comes back into the same medium after reflection from interface (Fig. 17). This phenomenon is called total internal reflection (TIR).
Fig. 17
113270.png, where μ RarerμDenser

Conditions for TIR

  • The ray must travel from denser medium to rarer medium.
  • The angle of incidence i must be greater than critical angle C

Dependence of critical angle

  • Color of light (or wavelength of light): Critical angle depends upon wavelength as 113277.png.
  • λR > λV CR > CV
  • sin C 113286.png (for two media)

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