Answer :

If h1 and h2 are the sizes of the images formed in two conjugate positions, then, the size of object is given as h = √h1h2 = √9×1 = 3cm.


The size of the image in the two conjugate positions is


In the first case, if the image is formed, the magnification is given as . Such that, v = 3u


Also, v = 20+u, since “v” and “u” interchange the conjugate positions.


3u = 20 + u


Or 2u = 20


Or u = 10


And, v = 20+ u = 20+10 = 30 cm


Now, focal length is calculated as :






Thus, correct answer is option (d)

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