Rainbows are arc shaped due to a simple geometrical principle. When the Sun shines after a shower, we often see an arc of beautiful colours in that part of the sky opposite to the Sun. If the rain has been heavy, the bow may spread all the way across the sky and its two ends seem to rest on the Earth below. The cause of this interesting phenomenon is the reflection and refraction of the Sun’s rays as they fall on drops of rain. As a ray passes into a drop of rain, the water acts as a tiny prism. The ray is bent, or refracted as it enters the drop and is separated into different colours. As it strikes the inner surface of the drop it is further refracted and dispersed. What we see in the heavens is a beautiful natural spectrum, produced by many drops.
There are seven colours (wavelengths) in each bow – violet, indigo, blue, green, yellow, orange and red. But these colours blend into each other so that the observer rarely sees more than four or five clearly. Each colour is formed by rays that reach the eye at a certain angle, (about 42 degrees for primary bow and 50 degrees for secondary bow) and the angle for a particular colour never changes. The higher the Sun the lower the bow and if the Sun is higher than 40 degrees, no bow can be seen. According to simple geometrical principles, the rain drops which lie at this particular angle and direction opposite to the Sun lie in the form of a full circle of a part of it (arc). Even if there are enough rain drops to form a full circle, to an observer on Earth it will look like an arc as it is limited by the horizon. When the Sun is near the horizon, an observer on a high mountain or in a balloon may see the whole circle of the rainbow.
