Where should an object be placed in front of a concave mirror of focal length 15 cm to have a magnification of 3 in case of
(a) real image
(b) virtual image ?

Explanation

The mirror equation is given by `1/v+1/u=1/f`
(a) In case of real image, `m=(-v)/u=-3`
So, v = 3u
`1/(3u)+1/u=1/(-15)`
`rArr4/(3u)=1/(-15)`
`rArru=-20cm`
(b) In case of virtual image, `m=(-v)/u=+3,v=-3u`
`-1/(3u)+1/u=1/(-15)`
`rArr2/(3u)=1/(-15)`
`rArru=-10cm`