Question

what is the use of combination of concave and convex lenses

Answer 1

The combination of Concave and Convex lenses are called Biconvex Lens.

Their uses are :-

Biconvex lens are used as a magnifying or condensing lenses. These are utilized in many of imaging systems such as the telescopes, monocular, microscopes, binoculars, cameras, projectors, etc. These lenses are also used as a burning-glass.