`+50` cm focal length bi-convex lens is recommended to correct the defect of vision of a man. Find the power of the lens.

The focal length of convex lens is 50 cm, its power is

The focal length of a convex lens is 20 cm. What is the power?

The focal length of a convex lens is 20 cm.What is the power?

a concave lens of focal length 20cm is placed in contact with a convex lens of focal length 40cm. Find the power of the combination.

The focal length of a lens is -50 cm. The nature and power of the lens is