An object is placed at 20 cm in front of a concave mirror produces three times magnified real image. What is focal length of the concave mirror ?

Options

A. 10 cm
B. 15 cm
C. 7.5 cm
D. 6.6 cm

Answer

B

Explanation

As image is real
`:.` Magnification, `m = - v/u= - 3 ` or `v = 3u`
Here, `u= - 20` cm
`:.v= - 60` cm
Accroding to mirror formula
`1/f = 1/v + 1/u`
`1/f= (1)/(-60)+(1)/(-20)= - (4)/(60)`
`f= - 15 cm`
`|f|=15 cm `