An object is placed at a distance of `1.5 m` from a screen and a convex lens is interposed between them. The magnification produced is `4`. What is the focal length of the lens ?

Answer

`0.24 m`

Explanation

Here, the image formed on the screen is real. Therefore, magnification `m = -4`. Let the lens be at a distance `x` from the object.
`:. U = - x, v = + (1.5 - x)`
As `m = (v)/(u) :. -4 = (1.5 - x)/(- x)`
`4x = 1.5 -x, x = 0.3 m`
`:. u = - 0.3 m` and `v = 1.5 - 0.3 = 1.2 m`
`(1)/(f) = (1)/(v) = (1)/(u) = (1)/(1.2) - (1)/(-0.3) = (5)/(1.2)`
`f = (1.2)/(5) = 0.24 m`