A point object is placed at a distance of 20cm from a thin plano-convex lens of focal length 15cm, if the plane surface is silvered. The image will form at :
60cm left of AB
30cm left of AB
12cm left of AB
60cm right of AB
A
30cm left of AB
B
60cm left of AB
C
12cm left of AB
D
60cm right of AB
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Solution
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Formula for focal length for a lens is
1f=(μ−1)(1R1−1R2) where f=15cm,R2=∞,R1=?,μ=1.5
Now solving the equation we have R1=15/2cm
After silvering of lens from one side the system starts acting as a mirror of focal length given by
1f=2μR2−2(μ−1)R1
⇒1v+1u=2μR2−2(μ−1)R1
⇒1v+1−20=2×1.5∞−2(1.5−1)×215⇒v=−12cm
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