Ans. OPTICAL FIBRE
Total internal reflection is used in fibre optics which has number of advantages in telecommunication field. Fibre optics consist of hair size threads of glass or plastic through which light can be travelled (Fig. 12.23). The inner part of the fibre optics is called core that carries the light and an outer concentric shell is called cladding. The core is made from glass or plastic of relatively high index of refraction. The cladding is made of glass or plastic, but of relatively low refractive index. Light entering from one end of the core strikes the core/cladding boundary at an angle of incidence greater than critical angle and is reflected back into the core. In this way, light travels many kilometres with small loss of energy.
In Pakistan, optical fibre is being used in telephone and advanced telecommunication systems. Now we can listen thousands of phone calls without any disturbance.