Question

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

(a) - 90 cm

(b) - 45 cm

(c) - 30 cm

(d) - 15 cm

Answer 1

Correct option is (d) - 15 cm

To find the object distance of a mirror having magnification + 2 and a radius of curvature of 60 cm we will follow the following steps:

As we know,

According to the question:

The concave mirror has a radius of curvature = 60 cm

So, Focal length = 60/2 = - 30

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

Magnification = 2

Also,

(Let it equation -1)

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

Magnification formula = -v/u

(Let it be equation -2)

Putting the value of magnification in the above formula in equation -2 we get,

-v/u = 2

v = - 2u

Now, putting the value of v and f in equation -1 we get,

u = - 15 cm