A convex lens is of focal length 10 cm. What is its power?

A convex lens of focal length 25 cm is kept is contact with a concave lens of focal length 40 cm. What is the power of their combination ?

In an optical instrument, a convex lens of focal length 20 cm is used in combination with a concave lens of focal length 40 cm. What is the power of this combination?

A convex lens of focal length 20 cm is placed in contact with a concave lens of focal length 10 cm. What is the focal length and power of the combination ?

A convex lens of focal length 10 cm is placed in contact with a concave lens of focal length 20 cm. What is the nature and focal length of the combination ?

A compound lens has been formed by joinined a convex and a concave lens, each of focal length 30 cm. what is the focal length of the combination? What will be the power ?

The dispersive powers of the materials of convex lens and concave lens placed in contact are in the ratio 1 : 2. For the achromatic combination, we take convex lens of focal length 100 cm. What should be the focal length of concave lens ?