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InterviewSolution
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An object is placed 30cm away from a convex lens of focal length 10cm and a sharp image is formed on a screen. Now, a convex lens is placed In contact with the convex lens. The screen now has to be moved by 45cm to get a sharp image again. The magnitude of focal length of the convex lends in (in cm)A. 72B. 60C. 36D. 20 |
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Answer» Correct Answer - D For the best first condition, `f_(1)=10cm u=-30cm` then, `(1)/(f_(1))=(1)/(v)-(1)(u) Rightarrow=(1)/(v)+(1)/(30)` `(1)/(v)=(1)/(10)-(1)/(30) Rightarrow v=(30)/(2)=15` For the second condition, when concave lens is placed, (where F=focal length of combination) `therefore(1)/(F)=(1)/(60)+(1)/(30)=F=(60)/(3)=20cm` The magnitude of focal length of concave lens, `(1)/(F)=(1)/(f_(1))+(1)/(f_(2)) Rightarrow (1)/(20)=(1)/(10)+(1)/(f_(2))` `f_(2)`=-20(negative sign for concave lens) |
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