Rainbows are caused when rays of light from the Sun hit water droplets which reflect some of the light back towards an observer. The water droplets are usually raindrops, but could also be water spray from a waterfall, a fountain, or even fog.
To be able to see a rainbow, you must have the Sun shining behind you and the water droplets in front of you.
It was the Greek philosopher Aristotle who first started contemplating rainbows and their colours, back in 350 BC.
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. In all of these cases two elements were essential for the characteristic burst of “rainbow” colour - water vapour or droplets and sunlight.
Our modern understanding of light and colour comes from Isaac Newton (1642-1726). He 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 are formed.
The colours of a rainbow are a spectrum - rather than separate standalone colours.
Newton thought it would be easier to discuss the structure of a spectrum (or rainbow), if it was broken down into simplified sections.
He also noted that the sequence of colours in a rainbow never changes, they always appear in the same order. It was Newton who coined the idea that there are seven identifiable colours in a spectrum (and rainbow).
Those colours are Red, Orange, Yellow, Green, Blue, Indigo and Violet.
The idea that there are seven colours in a rainbow remains today. You may think this is the case - but on closer inspection you will see that there are many more hues of colour than seven.
The colours of a rainbow are not a pure spectrum - it is actually made up of a multitude of individual spectral colours that have overlapped and mixed.
How are rainbows formed?
Rainbows are formed when sunlight is scattered from raindrops into the eyes of an observer.
The colours of the rainbow are caused by a combination of refraction (bending) and reflection (bouncing) of sunlight by water drops in the atmosphere.
Sunlight is made up of a spectrum of different colours that look white when we see them all mixed together. Light travels more slowly through water than air, so the light is bent as it enters a raindrop and becomes refracted, splitting the light into the spectrum of colours - forming a rainbow.
There are actually a number of different formations that a rainbow can take on - depending on the situation, these include: a double rainbow, a moonbow (sometimes called a lunar rainbow) or a fogbow. Read more about them on the Met Office website here.
The best time to see a rainbow?
The best time to catch a glimpse of a rainbow is just after the end of a storm.
Rainbows are more common in mornings and evenings.
To form a rainbow, sunlight needs to strike a raindrop at around 42 degrees. That’s unlikely to happen if the sun is higher than 42 degrees in the sky, according to the National Oceanic and Atmospheric Administration (NOAA).
The amount of the rainbow arc that is visible depends on how high the Sun is in the sky.
When the Sun is very high, you might see a rainbow that only just appears above the horizon. On the other hand, if you are lucky enough to see a rainbow from a plane or the top of a mountain you could see the whole arc.
Did you know? It is impossible for two people to see exactly the same rainbow.
The light bouncing off certain raindrops when one person sees a rainbow is bouncing off other raindrops from a completely different angle for someone else, according to LiveScience. This means that a different image is created depending on where you are stood. No two people can see the same rainbow because you can’t stand in exactly the same spot at exactly the same time to view the same arc.