Critical angle:

Critical angle: in case of propagation of light from denser to rarer medium through a plane boundary, critical angle is the angle is the of incidence fro which angle of refraction is 90˚.

Total Internal Reflection: If light is propagating from denser medium towards the rarer medium and angle of incidence is more than critical angle, then the light incident on the boundary is reflected back in the denser medium, obeying the laws of reflection. This phenomenon is called total internal reflection as total light energy is reflected; no part is absorbed or transmitted.

For total internal reflection.

(i)                  Light must be propagating from denser to rarer medium.

(ii)                Angle of incidence must exceed the critical angle.

Illustrations of total internal reflection

(i)                  Sparkling of diamond

(ii)                Mirage and looming.

(iii)               Shining of air bubble in water.

(iv)              Increase in duration of sun’s visibility – The sun become visible even before sun rise and remains visible even after sunset due to total internal reflection of light.

(v)                Shining of a smoked ball or a metal ball on which lamp soot is deposited when dipped in water.

(vi)              Optical Fibre: Optical fibre consists thousands of strands of a very fine quality glass or quartz (of refractive index 1.7) each strand coated with a layer of material of lower refractive index (1.5). In it, light is propagated along the axis of fibre through multiple total internal reflections, even though the fibre is curved, without loss of energy.

Applications:

(i)                  For transmitting optical signals and the two dimensional Pictures.

(ii)                For transmitting electrical signals by first converting them to light.

(iii)               For visualizing the internal sites of the body by doctors in endoscopy.