Question

A convex lens produces an inverted image magnified three times of an object placed at a distance of 15 cm from it. Calculate focal length of the lens.

 

Answer 1

Given:
Object distance, u =-15 cm
Magnification, m =-3 (Image is inverted.)
Applying magnification formula, we get:
m = v/u =-3
or, v/(-15) = -3
or, v = 45 cm
Applying lens formula, we get:
1/v - 1/u = 1/f
1/45- 1/(-15) = 1/f
1/f = ( 1+3 )/45
1/f = 4/45
or f = 11.2 cm
Hence, focal length of the lens is 11.2 cm.