Question

A 50 cm tall object is at a very large distance from a diverging lens. A virtual, erect and diminished image of the object is formed at a distance of 20 cm in front of the lens. How much is the focal length of the lens?

Answer 1

 Since the object is at infinity, the image will be formed at the focus. The image distance will be equal to the  focal length of the lens. Therefore, the focal length of the lens is 20 cm.