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 (b) 10 cm (c) 18 cm (d) 20 cm

 (b) 10 cm

Explanation

We know that a converging lens forms an image of the same size as the object when object is placed at a distance of $2f$ from the lens.

Here it is given that the image is slightly smaller than the object if the object is kept at a distance of 21 cm from the converging lens (beyond $2f$). Similarly, it is given that the image is bigger than the object if the object is kept at a distance of 19 cm (between $f$ and $2f$). Therefore, at 20 cm, the distance should be $2f$, which implies that the focal length will be approximately 10 cm $(\frac {20}{2})$.

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Updated on: 10-Oct-2022

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