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.
Who discovered the rainbow?
The Greek philosopher Aristotle first started musing about rainbows and their colours back in 350 BC. His ideas were picked up and elaborated upon by the Roman philosopher Seneca the Younger in his Book 1 of Naturales Quaestiones around 65 AD. 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). It is said that he actually only observed five colours, but added orange and indigo, to align the number of colours with the number of notes in the musical scale.
Most modern definitions of the spectrum of visible light exclude indigo, merging it into the ranges of shades of blue and violet.
The colours of a rainbow
The idea that there are seven colours in the rainbow still lasts to this day. At a 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.
The basic sequence for primary rainbows is always the same running from:
Red (the longest wavelength at around 780 nm) through to Violet (the shortest wavelength in the sequence at 380 nm).
The seven colour idea is still a popular one and it helps remember the order of the most recognisable colours in a rainbow. However, remember that there is also a whole range of colours, so many that we cannot distinguish them all with the naked eye.
