An object is placed at a certain distance from a convex lens of focal length 20 cm. Find the distance of the object if the image obtained is. magnified 4 times.

Explanation

Given data : Focal length of convex lens = 20 cm = f
To find : Magnification m = 4 ` = v/u`
Lens formula , ` 1/u - 1/v = 1/f`
` (1)/(4u) - (1)/(u) = 1/20 " " (1-4)/(4u) = (-3)/(4u)`
` (-3)/(4u) = 1/20`
` rArr 4u = -60`
` rArr u = (-60)/(4)`
` u = -15 cm`