Question

an object placed in centimetre in front of a lens has an image 20 centimetre behind the lens find the power of the lens

Answer 1

object distance, u = -10cm

image distance, v = +20cm

using lens maker formula, 1/v - 1/u = 1/f

or, 1/20 - 1/-10 = 1/f

or, 1/20 + 2/20 = 1/f

or, 3/20 = 1/f

f = 20/3 cm = 2/30 m = 1/15 m

so, power of lens , P = 1/f = +15D