# First Year Physics Optical Instruments

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$Example 4.2 Light passes from air into diamond with an angle of incidence of 45^{\circ} . Calculate the angle of refraction if the index of refraction of diamond is 2.42 .$

$An astronomical telescope having magnifying power of 5 consist of two thin lenses 24 \mathrm{~cm} apart. Find the focal lengths of the lenses.$

$(v) A convex lens is(a) thinner at the centre (b) thicker at the centre (c) a diverging lens (d) plane throughout$

$4.2 The speed of light in water is 2.25 \times 10^{5} \mathrm{~km} / \mathrm{s} . What is the index of refraction of water?$

$Example 4.6 Calculate the distance of the object from a convex lens having a focal length of 15 \mathrm{~cm} if the magnification is 3 and the image is real.$

$A converging lens of focal length 5.0 \mathrm{~cm} is used as a magnifying glass. If the near point of the observer is 25 \mathrm{~cm} and the lens is held close to the eye. Calculate (i) the distance of the object from the lens.$