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Question
A point object is placed at 20 cm from a thin plano-convex lens of focal length 15 cm whose plane surface is silvered. Locate the position and nature of the final image.
Solution
Given: u = - 20 cm
P = PL + PM + PL
`= 1/"f"_"L" + (- 1)/(("f"_"m")_(->∞)) + 1/"f"_"L"`
P = `2/"f"_"L"`
`(- 1)/"f" = 2/"f"_"L"`
f = `- 15/2`
Mirror formula,
`1/"f" = 1/"v" + 1/"u"`
`- 2/15 = 1/"v" - 1/20`
`1/"v" = (-2)/15 + 1/20`
`= (- 8 + 3)/60`
`= (-5)/60`
`1/"v" = 1/(-12)`
v = - 12 cm
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