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$4.4 Pick out true and false from the following sentences.(i) If a ray of light enters normally from one medium into the other then the angle of refraction is much larger than angle of incidence.(ii) One angle of the totally reflecting prism is 90^{\circ} while the other two angles are 45^{\circ} each.(iii). The principal focus of a concave lens is always virtual.(iv) A virtual image is that which can be obtained on a screẹ.(v) The reciprocal of the focal length of a lens is called its power.(vi) The unit of the power of the lens is dioptre.(vii) Centre of curvature is the centre of that sphere of which the lens is a part.(viii) The distance between the centre of curvature and the optical centre is called focal length.(ix) The image formed by a concave lens is always real.(x) The principal focus of a convex lens is always real. .(xi) Total internal reflection always takes place when the angle of incidence is much less than the critical angle.(xii) A simple microscope is a bi-concave lens.(xiii) The image in the human eye is formed at the iris.(xiv) A compound microscope contains a convex and a concave lens.$

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

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

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

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

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

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