The magnifications produced by a convex lens for two position of an object are 4 and 3, respectively. If the distance of separation between the two positions of the object is 2 cm , then the focal length of the lens is

A telescope has an objective of focal length 100 cm and an eye-piece of focal length 5 cm . The magnifying power of the telescope is

A convex lens and a concave lens, each with focal length of 4 cm are separated by a distance of 6 cm along their axis. An object is placed 8 cm before the convex lens. The distance between the object and its image is

If the image of an object is at the focal point $f$ to the right side of a convex lens, the position of the object on the left of the lens is at