what is the cause for the occurance of rainbow

Yes, a rainbow is the result of sunlight being refracted and reflected in individual raindrops. The raindrops act in the same way as a prism and bend the different wavelengths of white light differently, so that what we see are bands of different colours which are the components of white light. Each raindrop bends and refracts the light in the same way but we only see the colours from the raindrops when the light is directed exactly towards us. The bow is circular because of this (and because falling raindrops are usually nearly perfect spheres). If you were to be able to see the whole circle, you would find that the shadow of your head is always at the exact centre of the circle. One of the best pages that explains how rainbows happen is here: http://www.howstuffworks.com/question41.htm