Question

why concave lens is also known as diverging lens and why convex lens may also be used as magnifying glass

Answer 1

as concave lens diverges the light rays they are known as diverging lens.
convex lens will always give an enlarged image so that convex lens is used as magnifying glass

Answer 2

Concave lens is also known as diverging lens because it causes the light rays to bend away from its axis.

Convex lens can be used as magnifying glass bcoz it shows the shape of actual object very much bigger then it is.

hope it helps..!!