When an object is placed 20 cm in front of lens A, the image is real, inverted, magnified and formed at a great distance. When the same object is placed 20 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?
(a) The object is placed at focus, so f=20 cm of lens A.
(b) the object is placed at a distance twice the focal length, so f=10 cm of lens B.
(c) the nature of lens A is Convex lens (since image is real).
(d) the nature of lens B is Convex lens (since image is real).