You have studied that when a ray of light passes from a denser medium (glass) to a rarer medium (air), the refracted ray bends away from the normal and the angle of refraction is greater than the angle of incidence.

It is clear from Fig. 14.8 that as the angle of incidence (î) increases, the angle of refraction (ř) is always greater than (î), till for a particular value of angel of incidence, the corresponding angle of refraction becomes 90 degree  and the refracted ray grazes along the surface AB (fig. 14.8).

The angle of incidence in the denser medium for which corresponding angle of refraction is 90 degree in the rarer medium is called the critical angle. This angle of incidence is denoted by C.

If the refractive index of air with respect to glass is n, then in this case the ray OD is travelling from glass to air and is refracted in air therefore, by applying snell’s law, we will find the value of refractive index,

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Now for ray OD

Critical angle C is shown in fig. (14.8).

When the value of the angle of incidence becomes greater than the critical angel then the ray does not pass into the second medium (air) that is the ray of light no longer suffers refraction but all the rays having angle of incidence greater than the critical angel are totally reflected back in the denser medium (glass) obeying the laws of reflection. Such a reflection of light is called “total internal reflection”. It is clear from the above discussion that two conditions are essential for the total internal reflection.

1. The ray of light should travel from a denser medium to a rarer medium.
2. The angle of incidence should be greater than the critical angle.

Do you know?

In hot summer days, reflections of motor cars are seen on the roads and the image of an aero plane is seen on the runway, infect there is no water on the runway. It is due to total internal reflection known as mirage.