In an optical fibre

Explanation

Optical fibres too make use of the total internal reflection principle. Optical fibres consist of many thousands of very long fine quality, `"glass" // "quartz"` fibres. They are coated with a thin layer of a material of low refractive index. For example, the strand can be 0.0001 cm in diameter and the refractive index of the main fibre 1.7 and that of coating 1.5.

When the light is incident on one end of the fibre at a small angle, the light passes inside. It undergoes repeated total internal reflections along the fibre and finally comes out. The angle of incidence here is larger than the critical angle of the fibre material with respect of its coating. Even if the fibre is bent, the light can easily travel through along the fibre.
Uses of Optical Fibre
 (i) A bundle of optical fibres can be used as a .light pipe. in medical and optical examination called Endoscopy.
(ii) It can also be used for optical signal transmission,
(iii) Optical fibres have also been used for transmission and receiving electrical signals which are converted to light by suitable transducers. The main requirement is that there by very little absorption of light as it travels for long distances inside the optical fibres.
This has been achieved by purification and special preparation of materials such as quartz.