Question

An image in a convex lens is upright and magnified 3times. If the focal length of the lens is 15cm what is the object distance

Answer 1

Given

Focal length of the lens f = -15 cm ( according to sign convention)

Magnification m = 3

But magnification m = (-v)/u

-v = 3 u ------(A)

Again from lens formula we get,

(1/v) - (1/u) = (1/f)  

Reducing A in the above equation we get,

(-1/3u) - (1/u) = (-1/15)

(-1 +3)/(3u) = (-1/15)

u = (2×15)/3 = 10 cm