When sun and rain mingle

Subject: Science and Tech

Section: Physics

Why are rainbows arc shaped?

The cause of rainbow formation is 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 like 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.
Each colour is formed by rays that reach the eye at a certain angle, and the angle for a particular colour never changes.
The higher the sun the lower the bow. If the sun is higher than 40^o, no bow can be seen.
Raindrops that lie at this angle and opposite to the sun in the form of a full circle or a part of it. Even if there are enough rain drops to form a circle, to an observer on the 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 hot-air balloon may see the whole circle of the rainbow.

Primary and secondary rainbow:

Both the primary and secondary rainbows are formed by the reflection and refraction of sunlight in tiny water droplets.
When a sunbeam is being refracted twice and reflected once by the droplet, a primary rainbow will form. If the beam is being refracted twice and reflected twice, a secondary rainbow will form.
The primary rainbow forms between about 40° and 42° from the antisolar point.
- The light path involves refraction and a single reflection inside the water droplet.
The secondary rainbow is about 10° further out from the antisolar point than the primary bow, is about twice as wide, and has its colors reversed.
- As the secondary rainbow is formed by one more reflection than the primary rainbow, it is much fainter and rare to see.