Question

In optical fibre, which of the following principles is used?
A. Scattering
B. Refraction
C. Successive reflections
D. Total internal reflection

Answer 1

Hint: Study about the reflection and refraction and the relations between the angle of incidence and the angle of refraction. Try to understand what total internal reflection means and at what point it will happen. Learn about the optical fibres and about how they work.

Complete Step-by-Step solution:
Optical fibres are long thin strands of glass which diameter is about the diameter of a human hair. It consists of a glass core and a cladding outside the core which reflects the light back into the core. The outermost layer is a plastic coating to protect the fibre from any external damage.
The optical fibre uses the principle of total internal reflection. The light rays inside the optical fibre will have total internal reflection and they will pass through one end of the fibre to the other end.
When a light beam travels through a optically denser medium to a rarer medium a part of the light beam gets reflected to the denser medium and the other part is refracted to the rarer medium. This reflection is called the internal reflection.
Now if we keep on increasing the angle of incidence of the light beam at the interface of the two media after a certain angle refraction is not possible and the light beam is totally deflected. This process is called the total internal reflection.
This angle of incidence at which angle of refraction will be ${90}^0$ is called the critical angle. Only if the angle of incidence is greater than the critical angle than we can obtain total internal reflection.
So, the necessary condition for total internal reflection is that the angle of incidence should be greater than the critical angle.
Here the correct option is (D).

Note: For total internal reflection to happen the light beam should always travel from the denser medium to the rarer medium. Because when light beam goes from rarer to denser medium the refracted ray will go towards the normal at the interface and we will never get an angle of refraction of ${90}^0$ and hence we will not get total internal reflection.