Question

When an object is placed 10 cm in front of lens A, the image is real, inverted, magnified and formed at a great distance. When the same object is placed 10 cm in front of lens B, the image formed is real, inverted and same size as the object.
(a) What is the focal length of lens A?
(b) What is the focal length of lens B?
(c) What is the nature of lens A?
(d) What is the nature of lens B?

Answer 1

(a) The focal length of lens A is 10 cm as only a converging lens forms a real, inverted, magnified image at great distance when object is placed at the focus of the lens.
(b) The focal length of lens B is 5 cm as only a converging lens forms a real, inverted and same size image of object when the object is placed at 2F position of the lens.
(c) Lens A is a converging (convex) lens as only a converging lens forms a real image.
(d) Lens B is a converging (convex) lens as only a converging lens forms a real image.