Hello, Divya

Rainbow is a natural spectrum that occurs after a rainshower. It is a band of seven colours.

To a person, standing on the ground or even at heights, it appears as a semicircular band but if you are in a helicopter or an aeroplane, you will see that the rainbow is actually a full circle. 

When the sunrays fall on the droplets of water that are present in the atmosphere, the process of reflection and refraction takes place. The drops of water serve as the reflecting surface necessary for the rainbow formation. 

The white light, that is the sunlight (which consists of seven colours which are red, orange, yellow, green, blue, indigo and violet) when emerges from the sun and reaches the droplets of water by reflection and making an angle and then emerges out of the droplet by the process of refraction from another angle, thereby dispersing into the spectrum, that is the colourful band forms the rainbow. 

The colourful band is the rainbow consisting seven colours which are violent, indigo, blue, green, yellow, orange and red. There is an annotation for this - VIBGYOR. The order of the colours is red, orange, yellow, green, blue, indigo, violet but it appears to be in the opposite order to the human eyes. 

Hope this answer served you well!