The working of an optical fibre is based on which of the following phenomenon?

CONCEPT:

Total Internal Reflection (TIR):

• Total Internal Reflection (TIR): When a ray of light goes from denser to the rarer medium it bends away from the normal and as the angle of incidence in denser medium increases, the angle of refraction in the 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, light rays come back into the same medium after reflection from the interface. This phenomenon is called Total internal reflection (TIR).

Conditions for TIR: 

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

EXPLANATION:

Optical fibre:

• The working of an optical fibre is based on total internal reflection. Therefore option 2 is correct. 
• Optical fibres consist of many long high-quality composite glass/quartz fibres. Each fibre consists of a core and cladding.
• The refractive index of the core (μ1) material is higher than that of the cladding (μ2).
• When the light is incident on one end of the fibre at a small angle, the light passes inside, undergoes repeated total internal reflections along with the fibre and finally comes out.
• The angle of incidence is always larger than the critical angle of the core material with respect to its cladding.
• Even if the fibre is bent, the light can easily travel through along the fibre.