An object is placed at a distance of 15 cm from a concave mirror and its real image is formed at 30 cm in front of that mirror. The focal length of the mirror is

Options

A. `+ 20` cm
B. `-20` cm
C. `-10` cm
D. `+10` cm

Answer

C

Explanation

Here u = - 15 cm and v = - 30 cm, hence
`(1)/(f) = (1)/(v) + (1)/(u) = (1)/((-15)) + (1)/((-30)) = (-2-1)/(30) = - (1)/(10) rArr f = - 10` cm