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A screen is placed 90 cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 20 cm. Determine the focal length of the lens. |
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Answer» Solution :We can obtain two different positions of a convex lens to form the IMAGE of a GIVEN point object at another fixed point provided that values of u and v are interchanged in two positions. Therefore, as shown in Fig. 9.07, we have as shown in Fig, we have `Ol = 90 cm = |u|+ |v| =x+20 +x = 2x + 20` `rArr x= 35 cm` Hence, `u_(1) = -35 cm` and `v_(1) = +55 cm`  `therefore 1/f =1/v_(1) -1/u_(1) = 1/55 -1/(-35)` `=1/55 + 1/35 =(7+11)/385 = 18/385` or `f=385/18 = 21.4` cm |
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