Select a white coloured wall on which the sun rays fall. Stand in front of a wall in such a way that the sun rays fall on your back. Hold a tube through which water is flowing. Place your finger in the tube to obstruct the flow of water. Water comes out through the small gaps between the tube and your finger like a fountain. Observe the changes on the wall while the water shower is maintained; you can see colours on the wall.
• How is that you are able to see colours on the wall?
• Are the sun rays coming back to your eyes from the wall or from water drops?
Let us find out.
The beautiful colours of the rainbow are due to dispersion of the sunlight by millions of tiny water droplets. Let us consider the case of an individual water drop.

    Observe figure 12. The rays of sunlight enter the drop near its top surface. At this first refraction, the white light is dispersed into its spectrum of colours, violet being deviated the most and red the least. Reaching the opposite side of the drop, each colour is reflected back into the drop because of total internal reflection. Arriving at the surface of the drop, each colour is again refracted into air. At the second refraction the angle between red and violet rays further increases when compared to the angle between those at first refraction.

    The angle between the incoming and outgoing rays can be anything between 00 and about 420 . We observe bright rainbow when the angle between incoming and outgoing rays is near the maximum angle of 420 . Diagrammatically it is shown in figure 12.Although each drop disperses a full spectrum of colours, an observer is in a position to see only a single colour from any one drop depending upon its position

     If violet light from a single drop reaches the eye of an observer, red light from the same drop can’t reach his eye. It goes elsewhere possibly downwards of the eye of the observer (see in figure 13). To see red light, one must look at the drop higher in the sky. The colour red will be seen when the angle between a beam of sunlight and light sent back by a drop is 420 . The colour violet is seen when the angle between a sunbeam and light sent back by a drop is 400 . If you look at an angle between 400 and 420 , you