Describe the following : (i) Condition of total internal reflection within a fibre (ii) Acceptance angle (iii) Numerical aperture.

(i) Condition of total internal reflection within a fibre

Total internal reflection (TIR) is the phenomenon that involves the reflection of all the incident light off the boundary. TIR only takes place when both of the following two conditions are met:

1. The light is in the more dense medium and approaching the less dense medium.
2. The angle of incidence should be greater than the critical angle.

(ii) Acceptance angle

The light propagates through the fiber only when its angle of incidence at launching end is less than or equal to a certain value. This maximum possible angle with the axis of the fibre up to which a light ray accepted into the core of the fibre is called acceptance angle. The light rays that enter the fibre beyond acceptance angle refracts into cladding.

(iii) Numerical aperture

The sine of that acceptable angle (assuming an incident ray in air or vacuum) is called the numerical aperture, and it is essentially determined by the refractive index contrast between core and cladding of the fiber, assuming that the incident beam comes from air or vaccum:

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