﻿ An object and a screen are mounted on an optical bench and a converging lens is placed between them so that a sharp image is received on the screen. The linear magnification of the image is 2.5. The lens is now moved 30 cm nearer the screen and a sharp image is again formed on the screen. The focal length of the lens is    : Kaysons Education

# An Object And A Screen Are Mounted On An Optical Bench And A Converging Lens Is Placed Between Them So That A Sharp Image Is Received On The Screen. The Linear Magnification Of The Image Is 2.5. The Lens Is Now Moved 30 Cm Nearer The Screen And A Sharp Image Is Again Formed On The Screen. The Focal Length Of The Lens Is

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## Question

### Solution

Correct option is

14.3 cm

If O and I are the object and the screen respectively and L1 and L2 are the two conjugate positions of the lens (fig.), then

For lens at L1 we have u = –x and v = 30 + x

Linear magnification

or                          = 20 cm

Hence u = –20 cm and v = 30 + 20 = 50 cm.

Which gives                  f = 14.3 cm

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