Observe the refraction taking place when two prisms are kept together. Analysing the figure, prove that lens is a combination of prisms.

A prism is a transparent triangular piece of glass. It consists with plane (flat) refracting sides, an apex (top point) and a base (bottom). A ray of light incident to a prism is always bent towards the base of the prism. Here is light from the object bend toward the base of prism (AB).

So let consider two prisms placed base to base. Light rays through them coverage (come together) at some point on the other side of the prism. Look here -

In most of the cases, curved surface lenses are spherical (part of a sphere) and such lenses are called spherical lenses. This type of lens is also called plus lens.

Here is a complete ray diagram of two prism

We assume incident light 1 and 2 are coming from infinity so in the diagram both the ray was seen to convergent at a particular point. This is same as in convex lens i.e. when a ray of light from infinity strike to the lens converge at a point called focus. The same thing is happening here also so when these two prisms joint together and made thinner then this will act as a convex lens.

**Additional Information**:

To make concave lens we have to join two prisms from its apex.

Look here:-

As when incident ray falls to one of the side of prism it bend toward the base of a prism, making light to diverge (spread apart) on the other side of the prism. This type of lens is also called minus lens.