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

Question

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 (A) 20 cm (B) 40 cm (C) - 20 cm (D) 30 cm

Tardigrade Admin · Accepted Answer

m=-(v/u) (1/2) =-(v/60) v =-30 cm (1/f) =(1/v)-(1/u) (1/f) =(1/-30)+(1/-60) (1/f) =-(-2-1/60)=(-3/60) f=-20 cm

Q. An object is kept at a distance of $60 c m$ from a concave mirror. For getting a magnification of $\frac{1}{2}$ , focal length of the concave mirror required is

20 cm

40 cm

- 20 cm

30 cm

10 cm

$m = - \frac{v}{u}$
$\frac{1}{2} = - \frac{v}{60}$
$v = - 30 c m$
$\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$
$\frac{1}{f} = \frac{1}{- 30} + \frac{1}{- 60}$
$\frac{1}{f} = - \frac{- 2 - 1}{60} = \frac{- 3}{60}$
$f = - 20 c m$

Q. An object is kept at a distance of 60cm from a concave mirror. For getting a magnification of 21​, focal length of the concave mirror required is