how to find focal length of the coverging lens with the help of a mirror and torch?

Question

i am doing an experiment

there are two sharp images formed at different distance

Answer 1

Put the torch, or anything you can get an image of, at distance L1 ("object distance") from the lens. Measure the distance L2 ("image distance") of the focused image seen on a piece of paper on the other side. That's the "real" image. If you don't get an image, increase L1 until you do. Then solve:
reciprocal of focal length, 1/F = 1/L1 + 1/L2
If you have to use a mirror, place it right in back of the lens so the light makes two passes through the lens. This produces a result equivalent to one pass through a lens of focal length F/2 (except for the error due to the mirror not being at the exact location of the lens). Note that L1 and L2 are now on the same side of the lens. Have a piece of paper half-covering the torch. Adjust L1 for a focused image on the paper. F = L1. The reason is that, in the first setup, if you set L1 = 2F, then L2 = 2F also. In this case you have L1 = L2 = 2*(F/2), or F. See the ref. for lots of good lens info.