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$4.8 The distance between an object and a convex lens is 18 \mathrm{~cm} . The focal length of the lens is 6 \mathrm{~cm} . Determine the nature. position and magnification by using the lens formula.( -4.5 \mathrm{~cm} virtual 0.25 times)$  $Example 4.2 Light passes from air into diamond with an angle of incidence of 45^{\circ} . Calculate the angle of refraction if the index of refraction of diamond is 2.42 .$  $An astronomical telescope having magnifying power of 5 consist of two thin lenses 24 \mathrm{~cm} apart. Find the focal lengths of the lenses.$  $(v) A convex lens is(a) thinner at the centre (b) thicker at the centre (c) a diverging lens (d) plane throughout$  $4.2 The speed of light in water is 2.25 \times 10^{5} \mathrm{~km} / \mathrm{s} . What is the index of refraction of water?$  $Example 4.6 Calculate the distance of the object from a convex lens having a focal length of 15 \mathrm{~cm} if the magnification is 3 and the image is real.$  $A converging lens of focal length 5.0 \mathrm{~cm} is used as a magnifying glass. If the near point of the observer is 25 \mathrm{~cm} and the lens is held close to the eye. Calculate (i) the distance of the object from the lens.$  