A concave mirror has a focal length of 20 cm. Find the position or positions of an object for which the image-size is double of the object-size.

Explanation

Here, object distance, `u=?`, Focal length of concave mirror, `f= - 20 cm`, linear magnification, `m=2`
As `m = -v/u=2, :. v =- 2u`
As `1/u +(1)/(v)=(1)/(f), :. 1/u - (1)/(2u) = 1/-20` or `(1)/(2u)=1/-20` or `u =- 10cm`
Hence, the object is at 10 cm in front of the concave mirror.