Question

To obtain a magnification of +2 with a concave mirror of radius of curvature 60 cm the object distance must be ______.

पर्याय

• -90 cm

• -45 cm

• -30 cm

• -15 cm

Answer 1

To obtain a magnification of +2 with a concave mirror of radius of curvature 60 cm the object distance must be -15 cm .

Explanation:

Given the magnification +2.0 of a concave mirror of radius of curvature 60.0 cm .

Focal length =` 60/2` = -30

(-ve is taken because the focus of the mirror is behind the pole.)

Magnification, m`= (-v)/u`

Putting the value of magnification in the above formula we get,

`(-v)/u=2`

`v= -2u`

`v` is image distance ,`u` is object distance and `f` is focal length.

Since, 

`1/u + 1/v =1/f`

Putting te values 

`1/u + 1/(-2u) =1/(-30)`

`1/(2u) = 1/(-30)`

`u = -30/2cm`

`u =-15 cm`

Hence, the object distance is -15 cm.