Ask your friend to fill the vessel with water.

Look at the figure of arrow from the same position as before.

• What do you see now?

• Do you get an inverted image?

• How could this happen?

In the first case, when the vessel is empty, light from the arrow refracts at the curved interface, moves through the glass and enters into air then it again undergoes refraction on the opposite curved surface of the vessel (at the other end from where we are looking) and comes out into the air. In this way light travels through two media and comes out of the vessel and forms a diminished image.

In the second case, light enters through the curved surface, moves through water, comes out of the glass and forms an inverted image.

When the vessel is filled with water, there is a curved interface between two different media (air and water).

Assume that the refractive indices of both water and glass are the same (they really are not equal).

This setup of air and water separated by a curved surface is shown in figure 1.

• What happens to a ray that is incident on a curved surface separating the two media?

• Are the laws of refraction still valid?

Let us find out.

Consider a curved surface separating two different media as shown in figure 2

The centre of the sphere, of which curved surface is a part, is called as the centre of curvature. It is denoted by letter ‘C’.

Any line drawn from the centre of curvature to a point on the curved surface becomes normal to the curved surface at that point. The direction of the normal changes from one point to another point on the curved surface. The centre of the curved surface is called the pole (P) of the curved surface. The line that joins the centre of curvature and the pole is called ‘principal axis’.

• How do rays bend when they are incident on a curved surfaces