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  4. In an experiment a convex lens of focal length 15 cm is placed coaxially on an optical bench in front of a convex mirror at a distance of 5 cm from it. It is found that an object and its image coincide, if the object is placed at a distance of 20 cm from the lens. The focal length of the convex mirror is

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Q. In an experiment a convex lens of focal length $15\, cm$ is placed coaxially on an optical bench in front of a convex mirror at a distance of $5\, cm$ from it. It is found that an object and its image coincide, if the object is placed at a distance of $20\, cm$ from the lens. The focal length of the convex mirror is

Solution:

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
and $v =60\, cm$
$2 f =55$ and $f =27.5\, cm$