Total Internal Reflection

12.5 TOTAL INTERNAL REFLECTION

When a ray of light travelling in denser medium enters into a rarer medium, it bends away from the normal (Fig.12.9- a). If the angle of incidence 'i' increases, the angle of refraction 'r' also increases. For a particular value of the angle of incidence, the angle of refraction becomes $90^{\circ}$ . The angle of incidence, that causes the refracted ray in the rarer medium to bend through $90^{\circ}$ is called critical angle (Fig.12.9- b). When the angle of incidence becomes larger than the critical angle, no refraction occurs. The entire light is reflected back into the denser medium (Fig.12.9- c). This is known as total internal reflection of light.

12.6 APPLICATIONS OF TOTAL INTERNAL REFLECTION

Totally Internal Reflecting Prism

Many optical instruments use right- angled prisms to reflect a beam of light through $90^{\circ}$ or $180^{\circ}$ (by total internal reflection) such as cameras, binoculars, periscope and telescope. One of the angles of a right- angled prism is $90^{\circ}$ . When a ray of light strikes a face of prism perpendicularly, it enters the prim without deviation and strikes the hypotenuse at an angle of $45^{\circ}$ (Fig.12.10). Since the angle of incidence $45^{\circ}$ is greater than critical angle of the glass which is $42^{\circ}$ , the light is totally reflected by the prism through an angle of $90^{\circ}$ . Two such prisms are used in periscope (Fig.12.11). In Fig.12.12, the light is totally reflected by the prism by an angle of $180^{\circ}$ . Two such prisms are used in binoculars (Fig.12.13).

Optical Fibre