`color{brown} {(i) bbul{"Mirage:"}}` On hot summer days, the air near the ground becomes hotter than the air at higher levels.
The refractive index of air increases with its density. Hotter air is less dense, and has smaller refractive index than the cooler air. If the air currents are small, that is, the air is still, the optical density at different layers of air increases with height.
`color{blue} ✍️`As a result, light from a tall object such as a tree, passes through a medium whose refractive index decreases towards the ground. Thus, a ray of light from such an object successively bends away from the normal and undergoes total internal reflection, if the angle of incidence for the air near the ground exceeds the critical angle. This is shown in Fig. 9.14(b).
`color{blue} ✍️`To a distant observer, the light appears to be coming from somewhere below the ground. The observer naturally assumes that light is being reflected from the ground, say, by a pool of water near the tall object. Such inverted images of distant tall objects cause an optical illusion to the observer.
`color{blue} ✍️`This phenomenon is called mirage. This type of mirage is especially common in hot deserts. Some of you might have noticed that while moving in a bus or a car during a hot summer day, a distant patch of road, especially on a highway, appears to be wet. But, you do not find any evidence of wetness when you reach that spot. This is also due to mirage.
`color{brown} {(ii) bbul{"Diamond:"}}` Diamonds are known for their spectacular brilliance.
`color{blue} ✍️`Their brilliance is mainly due to the total internal reflection of light inside them. The critical angle for diamond-air interface `(≅ 24.4^o)` is very small, therefore once light enters a diamond, it is very likely to undergo total internal reflection inside it.
Diamonds found in nature rarely exhibit the brilliance for which they are known. It is the technical skill of a diamond cutter which makes diamonds to sparkle so brilliantly. By cutting the diamond suitably, multiple total internal reflections can be made to occur.
`color{brown} {(iii) bbul{"Prism:"}}` Prisms designed to bend light by `90^o` or by `180^o` make use of total internal reflection [Fig. 9.15(a) and (b)].
`color{blue} ✍️`Such a prism is also used to invert images without changing their size [Fig. 9.15(c)]. In the first two cases, the critical angle `i_c` for the material of the prism must be less than `45^o`. We see from Table 9.1 that this is true for both crown glass and dense flint glass.
`color{brown} {(iv)bbul{"Optical fibres:"}}` Now-a-days optical fibres are extensively used for transmitting audio and video signals through long distances. Optical fibres too make use of the phenomenon of total internal reflection. Optical fibres are fabricated with high quality composite glass/quartz fibres.
`color{blue} ✍️`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. 9.16).
`color{blue} ✍️`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.
`color{blue} ✍️`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. A bundle of optical fibres can be put to several uses.
`color{blue} ✍️`Optical fibres are extensively used for transmitting and receiving electrical signals which are converted to light by suitable transducers. Obviously, optical fibres can also be used for transmission of optical signals.
`color{blue} ✍️`For example, these are used as a ‘light pipe’ to facilitate visual examination of internal organs like esophagus, stomach and intestines. You might have seen a commonly available decorative lamp with fine plastic fibres with their free ends forming a fountain like structure.
`color{blue} ✍️`The other end of the fibres is fixed over an electric lamp. When the lamp is switched on, the light travels from the bottom of each fibre and appears at the tip of its free end as a dot of light.
`color{blue} ✍️`The fibres in such decorative lamps are optical fibres. The main requirement in fabricating optical fibres is that there should be very little absorption of light as it travels for long distances inside them.
`color{blue} ✍️`This has been achieved by purification and special preparation of materials such as quartz. In silica glass fibres, it is possible to transmit more than `95%` of the light over a fibre length of 1 km. (Compare with what you expect for a block of ordinary window glass `1` km thick.)
`color{brown} {(i) bbul{"Mirage:"}}` On hot summer days, the air near the ground becomes hotter than the air at higher levels.
The refractive index of air increases with its density. Hotter air is less dense, and has smaller refractive index than the cooler air. If the air currents are small, that is, the air is still, the optical density at different layers of air increases with height.
`color{blue} ✍️`As a result, light from a tall object such as a tree, passes through a medium whose refractive index decreases towards the ground. Thus, a ray of light from such an object successively bends away from the normal and undergoes total internal reflection, if the angle of incidence for the air near the ground exceeds the critical angle. This is shown in Fig. 9.14(b).
`color{blue} ✍️`To a distant observer, the light appears to be coming from somewhere below the ground. The observer naturally assumes that light is being reflected from the ground, say, by a pool of water near the tall object. Such inverted images of distant tall objects cause an optical illusion to the observer.
`color{blue} ✍️`This phenomenon is called mirage. This type of mirage is especially common in hot deserts. Some of you might have noticed that while moving in a bus or a car during a hot summer day, a distant patch of road, especially on a highway, appears to be wet. But, you do not find any evidence of wetness when you reach that spot. This is also due to mirage.
`color{brown} {(ii) bbul{"Diamond:"}}` Diamonds are known for their spectacular brilliance.
`color{blue} ✍️`Their brilliance is mainly due to the total internal reflection of light inside them. The critical angle for diamond-air interface `(≅ 24.4^o)` is very small, therefore once light enters a diamond, it is very likely to undergo total internal reflection inside it.
Diamonds found in nature rarely exhibit the brilliance for which they are known. It is the technical skill of a diamond cutter which makes diamonds to sparkle so brilliantly. By cutting the diamond suitably, multiple total internal reflections can be made to occur.
`color{brown} {(iii) bbul{"Prism:"}}` Prisms designed to bend light by `90^o` or by `180^o` make use of total internal reflection [Fig. 9.15(a) and (b)].
`color{blue} ✍️`Such a prism is also used to invert images without changing their size [Fig. 9.15(c)]. In the first two cases, the critical angle `i_c` for the material of the prism must be less than `45^o`. We see from Table 9.1 that this is true for both crown glass and dense flint glass.
`color{brown} {(iv)bbul{"Optical fibres:"}}` Now-a-days optical fibres are extensively used for transmitting audio and video signals through long distances. Optical fibres too make use of the phenomenon of total internal reflection. Optical fibres are fabricated with high quality composite glass/quartz fibres.
`color{blue} ✍️`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. 9.16).
`color{blue} ✍️`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.
`color{blue} ✍️`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. A bundle of optical fibres can be put to several uses.
`color{blue} ✍️`Optical fibres are extensively used for transmitting and receiving electrical signals which are converted to light by suitable transducers. Obviously, optical fibres can also be used for transmission of optical signals.
`color{blue} ✍️`For example, these are used as a ‘light pipe’ to facilitate visual examination of internal organs like esophagus, stomach and intestines. You might have seen a commonly available decorative lamp with fine plastic fibres with their free ends forming a fountain like structure.
`color{blue} ✍️`The other end of the fibres is fixed over an electric lamp. When the lamp is switched on, the light travels from the bottom of each fibre and appears at the tip of its free end as a dot of light.
`color{blue} ✍️`The fibres in such decorative lamps are optical fibres. The main requirement in fabricating optical fibres is that there should be very little absorption of light as it travels for long distances inside them.
`color{blue} ✍️`This has been achieved by purification and special preparation of materials such as quartz. In silica glass fibres, it is possible to transmit more than `95%` of the light over a fibre length of 1 km. (Compare with what you expect for a block of ordinary window glass `1` km thick.)