A small candle `2.5 cm` in size is placed `27 cm` in front of a concave mirror of radius of curvature `36 cm`. At what distance from the mirror should a screen be placed in order to receive a sharp image ? Describe the nature and size of the image. If the candle is moved closer to the mirror, how would the screen have to be moved ?

Explanation

(a) Using the relation, `(1)/(u)+(1)/(v)=(1)/(f)`
`(1)/(v)=(1)/(f)-(1)/(u)=(1)/(-18)=(1)/((-27))`
v=-54cm
The negative sign shows that the image is formed in front of the mirror, i.e., on the side of the object itself. Thus, the screen must be placed in a distance of 54 cm in front of the mirror.
(b) Magnification,m=`-(v)/(u)=(I)/(O)`
we get, `(I)/(2.5)=-((-54))/((-27))`
or `I=(2.5)xx(-02)=-5cm`
Thus clearly the image is real, inverted and magnified.