For an object placed at a distance $20 c m$ in front of a convex lens, the image is at a distance $20 c m$ behind the lens. The focal length of convex lens is:

20 c m

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10 c m

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15 c m

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40 c m

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Solution

The correct option is A $10 c m$
Answer is B.

There is a geometrical relationship between the focal length of a lens $f$ , the distance from the lens to the bright object $o$ and the distance from the lens to the projected image $i$ . The relationship between the distances is given as $\frac{1}{i} - \frac{1}{o} = \frac{1}{f}$ .
It is given that object distance is - $20$ cm and the image distance is $20 c m$ . So, the focal length is given as $\frac{1}{20} + \frac{1}{20} = \frac{1}{0.1}$ . The focal length $f = \frac{1}{0.1} = 10 c m .$

The focal length of the lens is 10 cm.

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Similar questions

For an object placed at a distance 20 cm in front of a convex lens, the image is at a distance 20 cm behind the lens. The focal length of convex lens is :

(a) 20 cm (b) 10 cm (c) 15 cm (d) 40 cm

An object is placed at a distance of 20 cm from a convex lens of focal length 10 cm. The image is formed on the other side of the lens at a distance:

An object is placed at 30 cm in front of the convex lens of focal length 10 cm. Find out the image distance from lens?

Q. A convex lens of focal length

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forms an image of height

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for an object situated at infinity. If a concave lens of focal length

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in front of convex lens the size image would be

Q. A

20 c m

tall object is placed perpendicular to the principal axis of a convex lens of focal length

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. If object distance is

15 c m

. Find the image distance.

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For an object placed at a distance 20 cm in front of a convex lens, the image is at a distance 20 cm behind the lens. The focal length of convex lens is:

For an object placed at a distance $20 c m$ in front of a convex lens, the image is at a distance $20 c m$ behind the lens. The focal length of convex lens is: