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  4. A source of light is located at double focal lengths from convergent lens. The focal length of lens is f =30 cm. At what distance from lens should a plane mirror be placed so that the ray reflected from mirror are parallel after passing through lens for second time ?

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Q. A source of light is located at double focal lengths from convergent lens. The focal length of lens is $f =30\, cm$. At what distance from lens should a plane mirror be placed so that the ray reflected from mirror are parallel after passing through lens for second time ?

Solution:

image
$2 x=30\, cm $
$x=15\, cm$
Distance of mirror from lens
$=30+ x =45\, cm$