Question

A convex lens of a focal length 20cm produces a real image 3 times magnified.Find the object distance

Answer 1

Explanation:

f = 20 cm m = -3 = v/u v = -3u Lens formula: 1/v - 1/u = 1/f 1/(-3u) - 1/u = 1/20 - 4/(3u) = 1/20 3u = - 4 x 20 u = -80/3 = - 26.6 cm The object is at a distance of 26.6 cm to the left from the optic centre of the lens

Answer 2

Answer: f= 20 cm

m=3 3=-v/u v=-3u

Explanation: from lens formula,

1/f =1/u+1/v

1/20= 1/u-1/3u

1/20=2/3u

3/40 =1/u

u=40/3=13.3 cm