Like refraction, total internal reflection has a wide range of applications.

1. Total reflecting prisms

A prism is a transparent triangular block, usually made of glass. Examples include the right angled prisms (corner angles: 45^o, 45⁰, 90^o) and the equilateral prism (each corner angle = 60^o). Total internal reflection can occur in glass prisms and this is widely used in prism binoculars, prism periscopes, erecting prisms, etc.

A periscope is an instrument used to see over obstacles; e.g. used by submarines to see over the surface of the sea. The figure below shows two constructions of the periscope; one using plane mirrors and the other using right-angled isosceles prisms.

With the plane mirror periscope, light is reflected downwards onto the second mirror which in turn reflects the light into the observer’s eye. This is shown in figure (a) above.

With the prism periscope, light passes straight through the surface RS because it meets the surface at a right angle. However the light hits the surface RT at an angle of incidence of 45^o which is greater than the critical angle of glass (about 42^o). So it undergoes total internal reflection as shown in figure (b) above. It is then totally internally reflected on the other prism onto the observer’s eye where he sees an erect image.

Total internal reflection in prism binoculars and erecting prisms is shown in the figures below;

A prism binocular is used to turn light through 180^o.

An erecting prism is used to invert an object. It can be shown by geometry that the angle of incidence along the horizontal surface is greater than the critical angle. Therefore total internal reflection occurs.

1. Optical fibres

These are thin flexible glass wires used in transmitting light signals. Total internal reflection continuously occurs as light moves along the fibres. Optical fibres are used in medicine and internal surgery to examine various body parts.

In communication, optical fibres are increasingly replacing copper cables. Some advantages of optical fibres over copper cables include the following;

• There is negligible cross talks between adjacent channels, thus information is more secure
• They have a greater information carrying capacity
• There is little loss of energy provided the angle of incidence is greater than the critical angle