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What are the colours of the rainbow?

The colours you see when a rainbow appears are the result of light being split into its various individual wavelengths.

This gives us a spectrum of colours that range from the shorter blue and violet wavelengths through to the longer red wavelengths. This sequence of colours gives us the characteristic pattern we're all familiar with, and that we learn from childhood through the use of mnemonic phrases.

The colours of the rainbow are Red, Orange, Yellow, Green, Blue, Indigo, Violet.

Who discovered the rainbow?

The Greek philosopher Aristotle first started musing about rainbows and their colours back in 350BC, and his ideas were picked up and elaborated upon by the Roman philosopher Seneca the Younger in his book 1 of Naturales Quaestiones around 65AD. Senaca was surprisingly ahead of his time in his reasoning, even predicting the discovery of the prism effect by Newton centuries later.

Throughout the ages, thinkers, philosophers and naturalists examined the phenomenon of the rainbow effect, noting its appearance not just in the sky but in other circumstances too.

But in every case two elements were essential for that characteristic burst of colour - water vapour or droplets and sunlight. Finally, Isaac Newton proved that white light is made up of a spectrum of colours by splitting light with a prism. His discovery, together with the work of others before him, finally explained how rainbows form.

He also noted that the sequence of the colours of a rainbow never changed, always running in the same order - He coined the idea that there are seven colours in a spectrum: red, orange, yellow, green, blue, indigo and violet (ROYGBIV).

The colours of a rainbow

The idea that there are seven colours in the rainbow still lasts to this day, and at a quick glance you might think this to be true, but closer inspection of a rainbow shows that there are far more than just seven individual hues.

A rainbow is not a pure spectrum. It is actually made up of a myriad of individual spectral colours that have overlapped and mixed as shown in this image.

Rainbow (middle: real, bottom: computed) compared to true spectrum (top): unsaturated colours and different colour profiles. Diagram: Andys

The basic sequence for primary rainbows is always the same running from:-

Red (the longest wavelength at around 780nm) through to Violet (the shortest wavelength in the sequence at 380nm).

The seven colour idea is still a popular one and is helpful for remembering the order of the most recognisable colours in a rainbow but remember that there are also a whole range of colours, so many that we cannot distinguish them all with the naked eye.

How to remember the colours of the rainbow

From a very early age we're taught how to remember the colours of the rainbow using what is known as a mnemonic.

This is a phrase that takes the first letter of each colour and makes up a new word which, in turn, creates a phrase that's easy to remember.

One of the traditional mnemonics is Richard Of York Gave Battle In Vain, but it's easy to make up one that's relevant to you.

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