A 3 cm tall object is placed 18 cm in front of a concave mirror of focal length 12 cm. At what distance from the mirror should a screen be placed to see a sharp image of the object on the screen. Also calculate the height of the image formed.

Explanation

`h_1` = +3 cm, f = -12 cm, u = -18 cm, v = ?, `h_2` = ?
`1/f = 1/v + 1/u`
`implies 1/v=1/f-1/u = (1)/(-12cm)-(1)/(-18 cm)`
`:. v = -36 cm`
`m=(h_2)/(h_1)= - v/u`
`:. h_2= -h_1 xx v/u= -3cm xx (-36cm)/(-18 cm)` = -6 cm