# A convex lens of focal length 10 cm forms a real image of an object placed at a distance of 20 cm from it. Midway between the convex lens and the position of the image a thin concave lens is introduced. The image formed now is at a distance of 5 cm away from the earlier position. What is the focal length of the concave lens?(adsbygoogle = window.adsbygoogle || []).push({});

I solved it in the following way:

Case-1

Let u, v and f be the object distance, image distance and the focal length of the convex lens.

Here u = 20 cm, f = + 10 cm

1/v = 1/f – 1/u

1/v= 1/10 – 1/20

v = 20 cm

Case- 2

Let u, v and f be the object distance, image distance and the focal length of the concave lens.

Here u = - 10(virtual object), v = + 15 cm

1/f = -1/10 + 1/15

f = - 30 cm

But the answer given in my book is – 5 cm.

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# Homework Help: Focal length of concave lens using convex lens

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