Question

How many foci does a lens have?

Answer 1

Hint: The focus of a lens is a point or a plane where the light rays that come from infinity meet or the light appears to meet at this point. One of the ways to find out the focus of a lens is by passing a parallel beam of Light and to notice where it converges or seems to originate from.

Complete answer:
A typical lens has two foci on either side of the lens
For example, the convex lens in this a parallel beam of light if projected on the lens bends itself on a fixed point this point is known as the focus of the lens.
The focus of a concave lens is that point on the principal axis on which the parallel rays of light that get diverged after refraction through the lens, when produced backwards, tend to meet at this particular point.
In simple terms for convex lenses the rays meet directly at this point and for concave lenses the rays meet when we produce them backwards.

Note:
Unlike lenses, concave mirror and convex mirror both have only one focus. For a concave mirror the light rays directly meet at the point of focus after getting reflected by the mirror and for the convex mirror the light rays seem to meet at the virtual focus when the rays that are reflected by the mirror are produced in backward direction. This is the main difference between mirror and lenses.