When the physis teacher has given a task to determine the focal length of a concave mirror a clever boy in the class focussed the mirror to a distant object. He moved the screen in fornt of the mirror and caught a real image on a screen at a distance of 20 cm from the mirror and he determined the focal length. Where should the object be place so that the object and the image are formed at the same position?

Explanation

The light rays coming from an object at infinity after reflection focus at the principal focus.
Hence, focal length f=20cm
The image is formed at the same position as that of the object, when the object is placed at the centre of curvature.
`:.` Radius of curvature `=2f=2xx20=40m`