A rainbow is a familiar sight in the sky when it rains on planet Earth, but what causes this spectacularly colourful phenomenon?
Put simply, a rainbow is caused by interaction between sunlight and atmospheric water. This most commonly means raindrops, but mist and spray can also cause rainbows.
When a beam of sunlight hits the surface of a raindrop, some of it is reflected while some of it passes inside, and is refracted as it does so. That refracted light then hits the rear of the raindrop.
Again, some of the light passes out of the raindrop and is refracted, but some of it is reflected back to the surface where it originally entered.
And so it continues, with some of that light in turn passing out of the drop while some is internally reflected, and so on.
It's the refracted light that we’re interested in.
Rainbow and refraction
As anyone who studied GCSE Physics or stared at the cover of Pink Floyd’s ‘Dark Side Of The Moon’ album can tell you, refracted light is split into a spectrum of colours – colours which, in the case of a rainbow, causes a semicircular arc to appear in the sky.
A rainbow appears curved because every drop of rain or mist is spherical, so the light it reflects/refracts effectively casts a circle of light on the sky.
At the edges of this circle the earlier refraction comes into play, with the most-refracted (red) light appearing at the circle’s outermost edge, and less-refracted colours (green or cyan) visible a little closer to the centre.
That’s why, when you see a rainbow, the red light is always at the top of the arc, and the violet light is always at the bottom.
This scattering and refraction of light is also key to why the sky is blue and the science of spectroscopy.
Rainbows and optical illusions
When you talk about seeing a rainbow, it’s worth pointing out that the operative word there is “you”.
Your eye perceives a rainbow thanks to the refraction, reflection and dispersion of light, but more precisely, thanks to the refraction, reflection and dispersion of the specific photons of light that have made it all the way from the Sun to your retina.
Your friend standing next to you has a completely different set of photons hitting his/her/their retinas.
So while you both may see a rainbow, you won’t see the same one: no two people on Earth ever have, or ever will!
Similarly, this is why rainbows don't actually touch the ground, and why you can't find the end of a rainbow: they're optical illusions caused by light being refracted by water.
You can also find rainbows at night: they're caused by moonlight and are known as moonbows.
