1.

A convex lens of focal length 20cm and a concave lens of focal length f are mounted coaxially 5cm apart. Parallel beam of light incident on the convex lens emerges from the concave lens as a parallel beam. Then, f in cm isA. 35B. 25C. 20D. 15

Answer» Correct Answer - d.
Clearly, power of system is zero.
`:.0=(1)/(20)+(1)/(f)-(5)/(20f)`
or `-(1)/(20)=(15)/(20f) or f=-15cm`


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