Question

what is lens ? and definition of concave and convex lens

Answer 1

A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction.

Answer 2

Answer:

A lens is a piece of glass or other transparent material with curved sides for concentrating or dispersing light rays, used singly (as in a magnifying glass) or with other lenses (as in a telescope)

Concave lens

A lens made by joining the parts of two spherical surface that is thinner in the middle than at the edges it converts the incident parallel rays and bends away from the principal axis. hence it is a diverging lens

Convex lens

A lens made by joining the parts of two spherical surface that  is thicker in the middle than at the edges. it converts the incident parallel rays and bends towards the principal axis. hence it is a converging lens