2015 Dec

Refraction at Work

December 2, 2015

Optic Fibre Clothes

High speed Internet, once considered a privilege, has now become a household thing across many countries. The cables used for transmission of Internet, connecting continents, are called as optical fiber cables (OFCs). These cables work on the principle of total internal reflection. Don’t gobble the bait and assume its reflection, unlike what the name suggests it’s a refraction phenomenon and its nature is detailed below.

Light bends as it moves from one medium to the other and this phenomenon is called as refraction. Let’s assume that light is shone across a surface of liquid, due to the difference in the nature of liquid and air, light bends. Light can bend away from the surface of the liquid (interface between two media) or towards the surface of the liquid. The path light chooses in the new medium depends on a property called as refractive index. Snell’s law states that if the refractive index of the new medium is low (compared to the earlier medium) then the light bends towards the interface where as if the refractive index of the new medium is high the opposite would happen. The angle by which the light ray bends is regulated not only by the refractive indices of the corresponding media but also the angle at which light is shone (technically called as angle of incidence). So in a situation where light ray goes from a high refractive index medium to low refractive index medium, the light ray bends towards the interface of the two media and the angle by which it bends is regulated by angle of incidence. As the angle of incidence is gradually increased, light ray bends more close to the interface (as per Snell’s law) and there reaches a point where the light ray travels parallel to the interface, and this particular angle of incidence where the light ray travels parallel to the surface is called as critical angle. As your wild thought suggests, if the light beam is shone at an angle larger than critical angle, the light beam bends back into the old medium and this phenomenon is called as total internal reflection. The energy loss during the process of total internal reflection is zero since all the photons associated with the light bounce back into the previous medium. Therefore during our Physics tuition classes, we have also discussed that this is one of the advantages that optical fibers have, over copper cables during the transmission of information.

In the case of a model optical fiber cable, there is a sheath made of material with low refractive index covering a long wire of high refractive index. If light is shone on the inner surface, the light reflects off the sheath and bends back into the old medium and this process repeats as depicted in the picture. And light beam can be sent from one end to the other without any loss of energy. Since light travels with a very high speed (order of 108 meters per second) information can be transferred across long distances in a split second.

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