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Question

If an object is placed at a distance of 30 cm and the image is formed at a distance of 10 cm on the same side of the lens, then find the focal length of the lens used.
  1. 10 cm
  2. 15 cm
  3. 20 cm
  4. 25 cm


Solution

The correct option is B 15 cm
Given:
Image distance, v=10 cm
Object distance, u=30 cm
[Using the sign conventions, as both object and image are on the left side of the lens] Let focal length be f.

Using the lens formula:
1v1u=1f
110130=1f
f=15 cm
Here negative sign shows that the lens is concave.

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