A rainbow is a colourful bow or arc of light that appears in the sky after or during rain. A rainbow is one of the prime examples of various physical phenomena such as reflection, refraction, and dispersion of light. It also helps to verify that the sunlight is not white in colour, but consists of a spectrum of colours having different wavelengths. To understand how a rainbow is formed, one must have a basic understanding of light. The light obtained from the sunlight appears to be white in colour, but it contains a number of colours. The speed of light is approximately equal to 3 x 10^8 m/sec. The light radiations emitted by the sun are arranged according to their respective wavelengths and are represented with the help of an electromagnetic spectrum. The gamma radiations emitted by the sun have the highest wavelength and least frequency, while radio waves have the lowest wavelength and highest frequency. The light waves existing within the wavelength range 380-740 nanometres are visible to the human eye. This band of light is known as white light.
Formation of a Rainbow
The formation of a rainbow is a meteorological phenomenon. The basic process of rainbow formation includes a light wave that gets split into multiple colours after passing through a water droplet. Here, each water droplet of the rain shower act as a small prism. In detail, the formation of a rainbow can be described as a three-step process. The first step starts with the light wave striking the face or the front surface of the raindrop. This causes the wave to undergo the dispersion phenomena and get split into seven different colours. The dispersed wave is also subjected to refraction due to the change in medium. The second step of rainbow formation is the total internal reflection of the wave, which takes place when the refracted wave proceeds further and strikes the back surface of the raindrop. This causes the spectrum of light to escape the raindrop and emerge out. The third and final step involves refraction of the light wave due to a change in medium.
Colours in a Rainbow
A rainbow consists of seven colours that are arranged with respect to their wavelengths. The red colour appears on the top or on the outer perimeter of the rainbow because it gets refracted at a steeper angle as compared to the other colours. Similarly, the violet colour is seen on the bottom or on the inner boundary of the rainbow as it tends to experience the least steep angle of refraction out of all the colours. If the colours present in a rainbow are seen from the bottom to the top, then the set of seven colours is abbreviated as VIBGYOR. Here, V stands for violet, I stands for indigo, B means blue, G represents green, Y signifies yellow, O is for orange, and R means the red colour.
Viewing Conditions of a Rainbow
To get a clear view of a rainbow one must ensure that the following conditions are properly met:
1. The back of the viewer must face the sun.
2. The angle of elevation must be more than or equal to 40 degrees.
3. For a better view, the person must stand under the sky between the sun and the rain shower at a location that has minimum obstacles.
Why are Rainbows Curved?
A rainbow appears to be a half or semi-circle when seen from the ground; however, in reality, a rainbow is a complete circle of seven colours. This can be observed easily while looking at a rainbow from an aeroplane. The bottom half of the full rainbow gets blocked due to the ground and only the top half curve of the circle is visible, which is why rainbows appear to be curved. The refraction, dispersion, and total internal reflection phenomena responsible for the formation of a rainbow happens all across the horizon. Most of the refracted white light gets scattered in the atmosphere and is not visible to the human eye. Only the light that is bent at an angle of 40 or 42 degrees is perceivable. The angle formed between the source of light, i.e., the sun, the observer, and the water droplets is majorly responsible for the curved or semi-circle shape of the rainbow. The low angle of the sun causes the rainbow to be more curved, while the high angle of the sun is responsible for the formation of a shallow rainbow.
A moonbow is a rainbow that usually emerges out during the night. Moonbows are observed when the moon is almost completely illuminated, i.e., the brightness level of the moon is approximately 85 per cent. The moon rarely gets lit up to this level, which means that the chances of viewing a moonbow are very low. The physical process of formation of a moonbow is similar to that of a rainbow. The only difference is that a moonbow makes use of moonlight as the illumination source rather than using the sunlight. Moonbows can be observed even in absence of a rain shower. In such a case, the water droplets of melting hail and fog or the mold spores serve to be the prism that split moonlight into a multicoloured curve.