question_answer

What happens inside optical fibre?

A)  Diffraction

B)  Polarization

C)  Interference

D)  Total internal reflection

Correct Answer: D

Solution :

An optical fibre is a device based on total internal reflection by which a light signal can be transferred from one place to the other with a negligible loss of energy. It consists of a very long and thin fibre of quartz glass (refractive index\[=1.7\]). Each fibre is of thickness nearly\[{{10}^{-4}}\,\,cm\]. The fibre is coated all around by a thin layer of a material (refractive index\[=1.5\]) rarer than the quartz glass. When a light ray is incident at one end A of the fibre making a small angle of incidence. It suffers refraction from air to quartz and strikes the fibre-layer interface at an angle of incidence greater than the critical angle. It therefore, suffers total internal reflection and strikes its opposite interface. At this interface, it again suffers total internal reflection. Thus, the ray reaches the other end B of the fibre.