Let us assume a system that consists of two lenses with focal length`f_1 and f_2` respectively. How do you find the focal length of the system experimentally, when?
two system are touching each other.

Two lenses of power -1.75D and +2.25 D respectively, are placed in contact. Calculate the focal length of the combination.

To minimize spherical aberration two lenses of focal lengths f_(1) and f_(2) are placed with a distance of separation equal to

In Ramsden eyepiece, the two planoconvex lenses each of focal length f are separated by a distance 12 cm. The equivalent focal length (in cm) of the eyepiece is

What is the focal length of a convex lens of focal length 30 cm in contact with a concave lens of focal length 20 cm ? Is the system a converging or a diverging lens ? Ignore thickness of the lenses.

A convex lens and concave lens of focal length +20 cm and -30 cm respectively . Then find the power of each lens. Find the focal length of combined lens. And find the combined power of the lenses (+5D,-3.3D, +60 cm , + 1.7 D)

A compound microscope consists of an object lens of focal length 2 cm and an eyepiece of focal length 5 cm. When an object is placed at 2.2 cm from the object lens, the final image is at 25 cm from the eye lens. What is the distance between the lenses ? What is the total linear magnification ?