Ans. TOTALLY INTERNAL REFLECTING PRISM
Many optical instruments use right-angled prism to reflect a beam of light through 90° or 180° (by total internal reflection) such as cameras, binoculars, periscope and telescope. One of the angles of a right-angled prism is 90°. 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° (Fig.12.22-a). Since the angle of incidence 45° is greater than critical angle of the glass which is 42°, the light is totally reflected by the prism through an angle of 90°. Two such prisms are used in periscope (Fig.12.22-b). In Fig.12.22-c, the light is totally reflected by the prism by an angle of 180°. Two such prisms are used in binoculars (Fig. 12.22-d).