Question

what is concave and convex lens ???

Answer 1

a concave lens is thinner at the middle. Rays of light that pass through the lens are diverged. hence it is a diverging lens.

A convex lens is a converging lens when a parellel rays of light pass through a convex lens the refracted ray converge at one point called principal focus.

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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