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Ray Optics and Optical Instruments Shine the beam from below the beaker such that it strikes at the upper water surface at the other end. Do you find that it undergoes partial reflection (which is seen as a spot on the table below) and partial refraction [which comes out in the air and is seen as a spot on the roof; Fig. 9.12(a)]? Now direct the laser beam from one side of the beaker such that it strikes the upper surface of water more obliquely [Fig. 9.12(b)]. Adjust the direction of laser beam until you find the angle for which the refraction above the water surface is totally absent and the beam is totally reflected back to water. This is total internal reflection at its simplest. Pour this water in a long test tube and shine the laser light from top, as shown in Fig. 9.12(c). Adjust the direction of the laser beam such that it is totally internally reflected every time it strikes the walls of the tube. This is similar to what happens in optical fibres. Take care not to look into the laser beam directly and not to point it at anybody’s face. 9.4.1Total internal reflection in nature and its technological applications (i) Prism: Prisms designed to bend light by 90° or by 180° make use of total internal reflection [Fig. 9.13(a) and (b)]. Such a prism is also used to invert images without changing their size [Fig. 9.13(c)]. In the first two cases, the critical angle i for the material of the prism must be less than 45°. We see from Table 9.1 that this is true for both FIGURE 9.12 Observing total internal crown glass and dense flint glass. reflection in water with (ii) Optical fibres: Nowadays optical fibres are extensively used for a laser beam (refraction transmitting audio and video signals through long distances. Optical due to glass of beaker fibres too make use of the phenomenon of total internal reflection. neglected being very Optical fibres are fabricated with high quality composite glass/quartz thin). fibres. Each fibre consists of a core and cladding. The refractive index of the material of the core is higher than that of the cladding. When a signal in the form of light is directed at one end of the fibre at a suitable angle, it undergoes repeated total internal reflections along the length of the fibre and finally comes out at the other end (Fig. 9.14). Since light undergoes total internal reflection at each stage, there is no appreciable loss in the intensity of the light signal. Optical fibres are fabricated such that light reflected at one side of inner surface strikes the other at an angle larger than the critical angle. Even if the fibre is bent, light can easily travel along its length. Thus, an optical fibre can be used to act as an optical pipe. FIGURE 9.13 Prisms designed to bend rays by A bundle of optical fibres can be put to 90° and 180° or to invert image without changing several uses. Optical fibres are extensively its size make use of total internal reflection. used for transmitting and receiving 231 Reprint 2026-27