• # question_answer Draw a ray diagram to show the image formation in a refracting type Astronomical telescope when the final image is formed at infinity. Write down the expression for its magnifying power, why should the diameter of the objective of telescope to larger? Or What is total internal reflection? Explain with the help of a diagram, how this principle is used for transmission of video signals using optical fibres.

Magnifying power (M) of a telescope point when final image is formed at infinity is ${{M}_{\infty }}=-{{f}_{0}}/{{f}_{e}}$ Ray diagram Here, ${{f}_{0}}$ is the focal length of objective lens, ${{f}_{e}}$ is the focal length of eyepiece. The diameter of objective is kept large to increase the (i) intensity of image (ii) resolving power of telescope. Or When a ray of light travels from denser $({{n}_{1}})$medium to rarer $({{n}_{2}})$ medium, if it incidents at the interface of two media at an angle greater than the critical angle for the two media, the ray is totally reflected back to denser medium, this phenomena is called Total Internal Reflection (TIP). For        $i={{i}_{c}},$ $r=90{}^\circ$ Substituting in equation             $\frac{\sin {{i}_{c}}}{\sin 90}=\frac{{{n}_{2}}}{{{n}_{1}}}\Rightarrow \frac{{{n}_{2}}}{{{n}_{1}}}=\sin {{i}_{c}}$ Transmission of video signals using optical fibre Optical fibre works on the principle of total internal reflection. It is a very long and thin pipe of quartz (n = 1.7) of thickness $\approx {{10}^{-4}}m$  coated all around with a material of refractive index = 1.5. A large number of such fibres are held together. When a light ray is incident on one end at a small angle of incidence, it undergoes refraction at the air quartz interface at an angle > critical angle and hence suffers total internal reflections. The light ray undergoes multiple total internal reflections and finally strikes the other end at an angle less than critical angle at quartz-air interface and emerges in air. Thus, the light signal is transmitted with no Toss of energy.