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 the convex lens is: - Noon Academy

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 the convex lens is:

Class 10 | ICSE | Physics | Refraction through a Lens | Selina

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 the convex lens is:

(a) 20 cm

(b) 10 cm

(c) 15 cm

(d) 40 cm

Solution:

The correct option is (b) 10 cm, the focal length of a convex lens is 10 cm.

Formula Used – 1/f = 1/v + 1/u

According to the question, v = 20cm and u = 20cm. So upon substituting values we get –

1/f = 1/20 + 1/20 = 2/20

Therefore, f = 10 cm

i

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