Lens: A lens is a piece of a refracting medium bounded by two surfaces, at least one of which is a curved surface. The commonly used lenses are the spherical lenses, which have either both surfaces spherical or one spherical and the other a plane one.

1.  A piece of glass or other transparent substance
with curved sides for concentrating or
dispersing light rays.
 The two main types
of lenses are:

2.  Convex lenses are thick in
the middle and thinner at
the edges.
 Convex lens is also
known as converging lens
as the light rays bend
inwards and converge at
a point which is known as
focal length.
 Convex lenses are used
to correct farsighted vision
problems.
 A concave lens is flat in
the middle and thicker at
the edges.
 Concave lens is also
known as diverging lens
because it bends the
parallel light rays outward
and diverges them at the
focal point.
 Concave lenses are used
to correct
nearsightedness.

4.  The most apparent
distinction between
mirrors and lenses are
that mirrors reflect
light rays (light
bounces back).
 It doesn’t allow light
rays to pass through it.
 Light rays are refracted
(pass-through) through
a lens.
 It allow light rays to
pass through it.