Question

A converging beam of light forms a sharp image on a screen. A lens is placed in the path of the beam at `10 cm` from the screen. It is found that the screen has to be moved `8 cm` further away from the lens to obtain a sharp image. Find the focal length and nature of the lens.

Answer 1

Correct Answer - `-22.5 cm`, concave
Here, `u = + 10 cm`
`v = + (10 + 8) = 18 cm`, Fig.
`(1)/(f)=(1)/(v)-(1)/(u)=(1)/(18)-(1)/(10)=-(8)/(180)`
`f = -(180)/(8) = -22.5 cm`
As `f` negative, the lens must be concave.
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