If an object is placed 21 cm from a converging lens, the image fo… - Vidyadip

MCQ

If an object is placed $21\ cm$ from a converging lens, the image formed is slightly smaller than the object. If the object is placed $19\ cm$ from the lens, the image formed is slightly larger than object. The approximate focal length of the lens is:

A
$5\ cm$
✓
$10\ cm$
C
$18\ cm$
D
$20\ cm $

✓

Answer

Correct option: B.

$10\ cm$

We know that a converging lens forms an image of same size as object when object is placed at a distance of $2f$ from the lens. It is given that the image is smaller than the object if object is kept at a distance of $21\ cm.$ Similarly, the image is bigger than the object if object is kept at a distance of $19\ cm.$ Therefore, at $20\ cm,$ the distance should be $2f.$ This means that the focal length is approximately $10\ cm.$

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