A diverging lens with magnitude of focal length 25 cm is placed at a distance of 15 cm from a converging lens of magnitude of focal length 20 cm . A beam of parallel light falls on the diverging lens.

Question

A diverging lens with magnitude of focal length 25 cm is placed at a distance of 15 cm from a converging lens of magnitude of focal length 20 cm . A beam of parallel light falls on the diverging lens. The final image formed is:

TestHub · Accepted Answer

Diverging lens forms the image at its focus. The image formed by diverging lens is the object for converging lens.

$∴$ Distance of the object from converging lens is $40 cm$ i.e., object is at double focal point.

$∴$ Image forms at double focal point on the other side of the lens.
$\frac{1}{v} - \frac{1}{4} = \frac{1}{f}$
$\frac{1}{v} - \frac{1}{(- 40)} = \frac{1}{20}$
$\frac{1}{v} + \frac{1}{40} = \frac{1}{20}$
$\frac{1}{v} = \frac{1}{20} - \frac{1}{40} = \frac{1}{40}$
$v = 40$

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