An object is kept at a distance of 60 cm from a concave mirror. For getting a magnification of `1/2`, focal length of the concave mirror required is

Options

A. 20 cm
B. 40 cm
C. `-`20 cm
D. 30 cm

Answer

C

Explanation

`u = - 60 cm, m = (-v)/(u) = (1)/(2)`
`rArr " " v = - 30 `cm (image is real )
`(1)/(v) + (1)/(u) = (1)/(f) `
`rArr " " (1)/(f) = - (1)/(30) - (1)/(60) = - (1)/(20) rArr f = - 20 ` cm.