Rainbows are seen in 2 shapes,namely
- Semi circular (Most of the times)
- Circular as a halo rainbow around the sun
Semi circular rainbow
To view this kind of rainbow one must have their back facing the sun and must be approximately positioned at an angle of 40 degrees w.r.t the rainbow formation.The sun rays falling on the water droplets which act as prism refract and reflect back the rays to your eyes.
Circular rainbow(Halo rainbow)
This kind of rainbow is optical phenomena formed due to millions of tiny ice crystals refracting the sunlight or moonlight. The radius of such rainbows is approximately 22 degrees.
Post scriptum: Suggestions are most welcome :)