Question

The image of an object is formed on itself when placed at a distance of 30 cm from concave mirror. What is the focal length of concave mirror?

Answer 1

Hi friend,

As the image is formed on object itself the object is paced on center of curvature of the mirror. Because when a object is placed on center of curvature of a mirror image is formed on object itself.
So Object distance=Radius of curvature (R)=2*focal length
                               =30=2f
                                f=15cm
Focal length of mirror is 15cm.

Answer 2

focal length is 15 cm