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A concave lens and a plane mirror re pla...

A concave lens and a plane mirror re placed coaxially 10 cm apart. A point object is placed at a distance of 40 cm from the concave lens. The final image formed by the combination is located 30 cm behind the mirror. Calculate the focal length of the concave lens.

Text Solution

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From figure. For the concave lens,
`"Object distance, "u=-40cm`
`"Image distance, "u=-(I_(1)M-PM)`
`" "=-(30-10)=-20cm`
`therefore" From lens formula"`
`(1)/(f)=(1)/(v)-(1)/(u)`
`=(1)/((-20))-((1)/(-40))`
`=(1)/(40)-(1)/(20)=-(1)/(40)`
`rArr" "f=-40cm`
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