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  4. A convex lens of focal length 20 cm placed in contact with a plane mirror acts as a

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Q. A convex lens of focal length $ 20\,cm $ placed in contact with a plane mirror acts as a

Punjab PMETPunjab PMET 2009

Solution:

When two lenses are kept in contact, the combined focal length of the combination is
$\frac{1}{F}=\frac{2}{f}+\frac{1}{F_{m}}$
For a plane mirror, $f_{m}=\infty$
$\therefore F=\frac{f}{2}=\frac{20}{2}=10\,cm$
Hence, arrangement behaves as a convex mirror of focal length $10\, cm$