What is a rainbow, and why the first colour in a rainbow is Red, not Blue?


rainbow is an atmospheric phenomenon which is caused by reflection, refraction and dispersion of sunlight from condensed water droplets resulting in a "spectrum of light" (a band of colours) appearing in the sky, that takes the shape of a multicoloured arc.


Due to dispersion, white light (sunlight) separates into different components of light by their different degrees of refraction according to wavelength.

It separates into a total of seven different colours, and the band of colours starts with red on the outside and changes through orange, yellow, green, blue, indigo to violet on the inside. 


In the visible spectrum, colours follow the order of their frequencies and the wavelengths. Red has the highest wavelength and least frequency, while violet has the least wavelength and highest frequency. It means that red is the least refracted wave (least scattered wave) while violet is the most refracted wave (most scattered wave).


Violet has the highest energy among visible light and red has the lowest. When sunlight hits drops of water, it is refracted in a cone-like manner, with the highest frequency light at the centre and fading to lower frequencies at the edges. Violet has the highest energy among visible light and red the lowest.


When sunlight hits drops of water, it is refracted in a cone-like manner, with the highest frequency light at the centre and fading to lower frequencies at the edges.

Hence, the start of the rainbow colour is the least refracted, then gradually changing up to the most refracted (violet). The order always remains the same because as the wavelengths going from longest to shortest, so we see the corresponding colours in this order: red, orange, yellow, green, blue, indigo and violet.


Process of rainbow formation

When a beam of sunlight enters the water droplet, it slows down and bends towards the normal, as it goes from the rarer medium (air) to denser medium (water), and then split into separate colours.

And, upon exiting the droplet, light speeds up and bends away from the normal, as it goes from the denser medium (water) to rarer medium (air).

The angle from the ground surface to the vertex of the rainbow (visual angle), is approximately 42, while the original sun rays enter approximately at 40


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Updated on: 10-Oct-2022

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