Light passing through convex and concave lens
A lens is an optical device which transmits and refracts light, converging or diverging the beam. If the lens is biconvex or plano–convex, a collimated beam of light travelling parallel to the lens axis and passing through the lens will be converged (or focused) to a spot on the axis, at a certain distance behind the lens (known as the focal length). In this case, the lens is called a positive or converging lens.
If the lens is biconcave or plano–concave, a collimated beam of light passing through the lens is diverged (spread); the lens is thus called a negative or diverging lens. The beam after passing through the lens appears to be emanating from a particular point on the axis in front of the lens; the distance from this point to the lens is also known as the focal length. The simplest use of a converging or convex lens is a magnifying glass.
When the object is far away enough to be outside the focal point of a converging lens, instead of a virtual image, a real image is formed. A diverging lens always produces an erect, diminished and virtual image irrespective of the position of the object.