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ex lens is ere, Yength of the, lens is 20 cm., , BSE. 1996) [Ans.-15 cm, -60 cin], ¢ lens of focal length 20 em and made, glass («= 1:5) is immersed in water of, we = 1:33. Calculate change in focal length of the, lens. ~ (P.S.E.B. 2001) [Ans. 58.23 cm], 31. A needle placed 45 cm from a lens forms an, image on the serecn placed YO cm on the other, side of the lens. Identify the type of the lens and —, determine its focal length. What is the size of the, image if size of needle is 5 -Qem ?, (P.S.E.B. 1996 S) [Ans. Convex, 30 cm, —10 cm], 32. The radius of curvature of cither face of a convex, lens is equal to its focal length. What is the, refractive index of the material ?, : [Ans. 4 = 1-5], 33. A convergent beam f light passes through a, diverging lens of foc. ength 0-2m and comes, to focus 0-3 m behind tuc lens. Vind the position, , of the point at which the beam would converge, in the absence of the lens., , (C.B.S.E. 1998) |Ans. 0-12 22], 34. The radii of curvature of the surfaces of a double, convex lens are 20 cm and:30 cm. What will be, its focal length and power in air and water, respectively ? Refractive indices for glass and, water are 3/2 and 4/3 respectively., , [Ans. 24 cm, 4°17 1D; 96 em, 1:04], 35, Calculate the distance at which an Object should, be placed in front of a convex lens of focal tength, , 10 cm to obtain an image of double ils size, ~, , [Ans. -5 cm, ~15 em}, 86. A biconvex lens is made of glass with « = 1-52, Tach surface has a radius of curvature cqual to, , 30 cm. An object of height 3 cm is placed 14 em