9.4.1. Total 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. Such a prism is also used to invert images without changing their size. 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 crown glass and dense flint glass.

Prisms utilize the principle of total internal reflection to bend light. When light enters a prism at an angle greater than the critical angle, it reflects completely inside the prism without passing through. The 90° or 180° bending effect is used in various optical instruments to manipulate how we see images, such as through binoculars. The critical angle is the angle of incidence above which total internal reflection occurs, and it needs to be lower than 45° for materials like crown glass and dense flint glass used in prisms.

Think of a prism like a smooth water slide at an amusement park – if you enter the slide at the correct angle, you glide down quickly without slowing down (total reflection). If you enter at a wrong angle, you might fall off the slide or slow down significantly, just like how light can escape instead of reflecting inside the prism.

(ii) Optical fibres: Nowadays 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. 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.

Optical fibres are designed to transmit light signals over long distances with minimal loss. They consist of two layers: a core with a higher refractive index and a cladding with a lower refractive index. When light enters the fibre at the right angle, it reflects internally multiple times instead of escaping, thus preserving the signal's integrity even over long distances. This feature of total internal reflection allows these fibres to bend and navigate corners while maintaining the signal quality.

Imagine a water slide that is very twisty. If the water (light) hits the sides of the slide (core) at the right angle, it keeps bouncing off and stays inside, allowing the water to flow faster without leaking out. This concept is similar to how optical fibres work to carry signals swiftly and efficiently, making them crucial for internet and telecommunication.

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. 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.

For optical fibres to function effectively, materials used must allow for minimal absorption of light. This means that once light enters the core, it should travel as far as possible without losing intensity. The improvements in material production, such as purifying quartz for better performance, have enabled modern optical fibres to transmit over distances of 1 km while retaining a high percentage of the transmitted light. This technology forms the backbone of modern telecommunications.

Think of optical fibres like a perfect vacuum tube where you’re sending marbles (light) through. If the tube is clear and smooth, the marbles can travel very far without slowing down or losing energy. However, if the tube had bumps or dirt, the marbles would get stuck or slow down. Similarly, keeping optical fibres clean and made from the right material ensures that signals travel efficiently.

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