If an object is placed at a distance of 10 cm in from of a plane mirror, how far would it be from its image?
Image distance from the mirror = Object distance from the mirror = 10 cm
So, distance of the object from its image = 10 cm + 10 cm = 20 cm
At what distance from a concave mirror of focal length 10 cm should an object be placed so that : (a) its real image is formed 20 cm from the mirror ? (b) its virtual image is formed 20 cm from the mirror ?