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 30 cm in front of the lens. How much is the focal length of the lens?
When an object is placed at a very large distance from a diverging lens, then image is formed at the focus of the lens.
Therefore, the focal length of the lens is 30 cm.