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A concave lens is kept in contact with a...

A concave lens is kept in contact with a convex lens of focal length 20 cm. The combination behaves as a convex lens of focal length 50 cm . Find the power of concave lens.

Text Solution

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`1/F = (1)/(f_1) + (1)/(f_2)`
`1/50 = 1/20 + (1)/(f_2) " or " f^2 = - 100/3 cm`
` P_2 = (100)/(-100/3)`
` D = - 3D`
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