A person wants a real image of his own, 3 times enlarged. Where should he stand in front of a concave mirror of radius of curvature of 30cm.

Explanation

As radius of curvature (R) is negative for a concave mirror, so R= -30 cm
The focal length of the mirror is
` f = R/2 = (-30cm)/(2) = -15cm`
As the person wants his real enlarged image, so magnification,
`m = -v/u = -3` or v = 3u
According to mirror formula
` 1/v + 1/u = 1/f therefore (1)/(3u) + (1)/(u) = (1)/(-15cm)`
`u = (-60cm)/(3) = -20cm`
where, -ve sign shows that the person should stand in front of the mirror.