A totally reflecting prism is that which has one of its angle equal to 90 degree and each of the remaining two angles equal to 45 degree here the phenomenon of total internal reflection is used. The critical angle of glass is 42 degree. if a ray of light strikes one of its faces perpendicularly, it enters the prism without any change of direction and meets the hypotenuse at an angle of 45 degree, so the angle of incidence is also 45 degree. As it is greater than the critical angle of the glass, the ray will be totally reflected, the angle of reflection being 45 degree. This reflected ray thus strikes the other face perpendicularly and comes out of the prism. The ray is thus deviated through an angle of 90 degree. Fig. 14. 10 (a) shows rays coming from the object AB are deviated through an angle of 90 degree and emerge out the prism as AB. Due to this fact the prism is used in periscope fig. 14. 10 (b) shows how a totally reflecting prism deviated the incident ray coming from the object AB through an angle of 180 degree and emerges out of the prism as A’ B’ after twice total internal reflections inside the prism. Due to this fact, this prism is used in “prism binocular” A prism binocular is made by using two telescopes separated at a distance equal to the distance between the two eyes.
In fig. 14. 10 © , this prism is used to erect the image of an object i.e., to invert the rays coming from an object. For example, such a prism placed in front of a projector will show an erect image on the screen of an inverted slide or film placed on the other end of the projector.
14. 11 OPTICAL FIBRE
In recent years an important and interesting application of principle of total internal reflection is used in optical fibres, as shown in fig. 14. 11, an optical fibre is made of a highly transparent fine strad of glass or plastic coated or cladded with another type of glass whose refractive index is less than the inner tube. Generally, the refractive index of inner tube is 1.52 and that of outer coating is 1.48.
If light rays enter at one end of an optical fibre at an angle of incidence greater than the critical angle then these rays undergo total internal reflection repeatedly at the walls and come out at the other end without any loss of intensity. Thus, light travels along the fibre no matter how it may be curved and comes out with the same intensity.
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If several thousands of fine strands of optical fibres are bundled together then a flexible light pipe is obtained which can be used by doctors and engineers to illuminate those inaccessible places which otherwise are not possible to examine.
Like other developed countries of the world, Pakistan is also using optical fibres in telecommunication and other modern communication systems. A single strand of optical fibre can transmit thousands of telephone calls at the same time without interfering each other. Optical fibres are highly flexible and light in weight and are much cheaper as compared to copper cables. Generally, a cable consists of a bundle of 30 optical fibres. The transmitting capacity of light pipes is thousands of times greater than the radio waves.
Endoscope is such an instrument which is used for viewing and photographing of the internal structure of human body by using instruments. A few of these are as follows. For example, gastroscope’ is an instrument to view the stomach. ‘Branchoscope’ is to examine the sore throat and ‘Cystoscope’ to examine liver. In endoscope by using a bundle of optical fibres, the affected part of the body is illuminated and it is viewed by another bundle of optical fibres. A video camera is fitted outside this bundle and this makes the interior visible to the surgeon which helps him to diagnose easily.
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