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A lens is a transparent optical medium bounded by two surfaces; at least one of which should be spherical. Applying the formula of image formation by a single spherical surface successively at the two surfaces of a thin lens, a formula known as lens maker's formula and hence the basic lens formula can be obtained. The focal length (or power) of a lens depends on the radii of its surfaces and the refractive index of its material with respect to the surrounding medium. The refractive index of a material depends on the wavelength of light used. Combination of lenses helps us to obtain diverging or converging lenses of desired power and magnification.
An object is placed in front of a converging lens. Obtain the conditions under which the magnification produced by the lens is (i) negative and (ii) positive.
Find the two possible positions of an object kept in front of a lens of + 5.0 D, so that the image formed in both cases is four times magnified.
An equiconvex lens forms a two times enlarged real image when an object is kept 16 cm from it. The lens is cut into two identical plano-convex lenses. If the object is again kept 16 cm in front of one of these lenses, then find the nature and position of the image formed.
A plano convex lens is made of glass of refractive index 1.5. The radius of curvature of the convex surface is 25 cm. (ii.i) Calculate the focal length of the lens. (ii.ii) If an object is placed 50 cm in front of the lens, find the nature and position of the image formed.
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