When an object is placed at a distance 60 cm from a convex spherical mirror, the magnification produced is `1//2`. Where should the object be placed to get a magnification of `1//3` ?

Options

A. -200 cm
B. +150 cm
C. - 120 cm
D. None

Answer

C

Explanation

Here, `u = -60 cm, m = 1/2`
As `m = - v/u, 1/2 = -v/(-60), v = 30 cm`
As `(1)/(f) = 1/u + (1)/(v), (1)/(f) = 1/(-60) + 1/30 = (-1+2)/60 = 1/60, f = 60 cm`
Again, `m' = -(v')/(u') or 1/3 = -(v')/(u'), v' = -(v')/3`
As `(1)/(f) = 1/(v') + 1/(u'), 1/60 = -3/(u') + 1/(u') = (-2)/(u')`
`u' = -120 cm`.