Natural phenomenon / 5 min read / 30 September 2021
Considered as one of the most beautiful of nature’s displays, rainbow can be described as a meteorological phenomenon resulting in a spectrum of light in the sky. The Sun must be behind and the clouds cleared away from the Sun for the rainbow to be visible.
How does it form?
The formation of a rainbow requires water droplets to be floating in air, that’s why we see them right after it rains. It involves a series of physical phenomena- reflection, refraction, dispersion and total internal reflection. The occurrence of each of these is due to the interaction of light with air and water and the boundaries between them.
When white light of the sun shines on a waterdrop, as it passes into the droplet, the light bends or refracts, because light travels slower in water than in air. It then splits into seven colors (dispersion of light) and is reflected at rear of raindrop (internal reflection). As the light exits the drop, it is refracted again. Is it shaped like an arc as it looks like? Unlike how it usually looks like, a rainbow is actually round like a circle; on the ground, the bottom part is hidden. From an airplane, in the right conditions, one can see the entire circle. It is circular because when a raindrop bends light, the light exits the raindrop at an angle 40-42 degrees away from the angle it entered the waterdrop.
The traditional description of the rainbow is that it is made up of seven colors- violet, indigo, blue, green, yellow, orange and red, commonly known by the acronym VIBGYOR.
There are several variations of rainbow. These include-
Double rainbow: Double rainbows are caused by light being used reflected twice inside the raindrop. The “second” rainbow is called secondary rainbow.
Higher-Order rainbows: A rainbow’s ‘order’ is its reflective number (for eg: Primary rainbows are first-order rainbows and secondary rainbows are second-order rainbows). Higher-Order rainbows appear to viewers facing both toward and away from the Sun.
Twinned rainbow: This includes two distinct rainbows produced from a single endpoint. Twinned rainbows are the result of light hitting an air mass with different sizes and shapes of water droplets.
Supernumerary rainbow: They are faint arc lines that appear inside the main rainbow.
Reflection rainbow: A reflection rainbow appears above a body of water. A primary rainbow is reflected by the water and the reflected light produces a reflection rainbow however, they do not mirror the primary rainbow, rather they often appear to stretch above it.
Reflected rainbow: Created by rays of light reflected by the water surface, after the rays have passed through water droplets, they appear directly on the surface of a water body.
Red rainbow also known as monochrome rainbow, Fogbow which is formed by light being reflected and refracted by fog and Moonbow, produced by light reflected by the Moon, are other variations.