A mirror of width 'l' m is kept on a wall. A person is approaching the mirror from the front. An observer is behind a screen has a view of the mirror as shown. How far from the mirror will be person be when the observer will be able to see him in the mirror.

$(a)\;l\;m \\ (b)\;\frac{l}{3}\;m \\ (c)\;\frac{l}{2}\;m \\ (d)\;2l \;m$

The field of view for the observer is only from point P.
Using geometry and the principle of reflection we get that the person is at a distance of $\large\frac{l}{2}$ from the mirror when the observer can see him.
Hence c is the correct answer.