When light rays travel from denser medium to a rarer medium, they bend away from the normal. This phenomenon in which the rays of the light bend is known as refraction of light. For a particular angle of incidence, the angle of refraction is 900. This angle of incidence for which the angle of refraction is 900 is known as the critical angle. When the angle of incidence is greater than the critical angle, refraction doesn’t occur, rather the entire light gets reflected back into the denser medium. The phenomenon that involves the reflection of all incident light off the boundary is known as total internal reflection.
Now that we know what is a total internal reflection, let us discuss how the principle is applied to real life.
Following are a few applications of total internal reflection:
Optical Fibres in Communication
Optical fibres are commonly used in the field of telecommunication. Fibre optics comprises hair-like strings made of plastic or glass through which light travels. The internal piece of the optic fibre is known as the core and the external shell is known as the cladding. The core is made of glass or plastic of high refractive index while the cladding is made of glass or plastic of a moderately low refractive index. When light enters from one end of the core and strikes the core-cladding boundary at an angle of incidence greater than the critical angle, the light propagates along with the fibre in a zigzag fashion undergoing a series of total internal reflection. This way light travels many kilometres with small loss of energy.
Total internal reflection explains why diamonds sparkle more than other materials. The critical angle for the diamond-to-air surface is only 24.4º. Hence, when light enters the diamond it has trouble getting out. The light freely enters the diamond but it can get out only if it makes an angle lesser than 24.4º. Good diamonds are very clear because light makes many internal reflections and is concentrated at only a few places where it can exit—hence the sparkle.
An endoscope is a clinical instrument that is utilized for exploratory diagnostics and surgical purposes. An endoscope is made of two fibre-optic tubes which are inserted into a pipe. The medical procedure utilizing an endoscope is known as endoscopy. The light enters through one of the fibre tubes and illuminates the organ of the patient. Then the light is transmitted back to the doctor’s review lens through the external fibre tube by internal reflection.
Prisms and Binoculars
Optical instruments such as cameras, binoculars, periscopes and telescopes employ right-angled prisms to reflect a beam of light through 900 or 1800. When a ray of light strikes a face of prism perpendicularly, it enters the prism without deviation and strikes the hypotenuse at an angle of 45o. As the critical angle of the glass is 42o, the light is totally reflected by the prism through an angle of 900. Two such prisms are used in binoculars, periscopes where the light is totally reflected by the prism by an angle of 1800.
Mirage is an optical illusion observed on a hot day in a desert. The observer sees an inverted image of objects such as a tree and is under the impression that there is a pool of water. This phenomenon is known as mirage. It occurs when light travels from optically denser medium (cool air) to a rarer medium (hot air) at an angle of incidence greater than the critical angle. The hot weather in the desert warms the layer of air just above the ground. When the light moves through cold air into the layer of hot air, it is refracted. The warm air near the ground refracts the light from the sky into a U-shaped bend. Our brain perceives this light to have travelled in a straight line. Our brain fails to recognise the image as bent light from the sky. Instead, our brain misinterprets the light to have come from something on the ground.
These were a few of the applications of total internal reflection. On the living planet, we call Earth, plenty of interesting events occur. At some point in time, your curiosity would have pushed you to ask questions about what’s going on. Electrolysis is a technique that makes use of direct electric current to initiate an otherwise non-spontaneous chemical reaction. Do you know the one important thing about electrolysis in everyday life? It is used in the production of metals such as Sodium and Calcium that are difficult to extract naturally. To know many more concepts like this that play a big role in our everyday life subscribe to BYJU’S.