You'll never see a straight rainbow on Earth. Here's the science behind the curved beams of light in our sky

Ever wondered why rainbows are always round? Put very simply, it’s because rainbows are an optical illusion that originates in droplets of water.

And because droplets of water are round, the rainbows they form are also round.

But now let’s dig into that a little deeper…

Rainbows are formed due to the interaction between sunlight and raindrops (though they can also form wherever there is a lot of airborne moisture, such as in the mist around a waterfall).

When sunlight encounters a raindrop, some of that light is reflected off the surface, and some passes into the body of the raindrop.

The latter light is refracted as it passes through the raindrop’s surface. Then, when it reaches the far side of the raindrop, the same thing happens.

Some light passes out through the back of the raindrop, and some is reflected internally.

And then the same thing happens again. Some of the light that has entered the raindrop, which has been refracted as it did so and bounced off the ‘back wall’, is now reflected internally for a second time.

But some of it passes out through the surface, is refracted once more, and eventually reaches our eyes.

The cumulative effect of all that reflecting and refracting is that the light reaching our eyes has been split into its component wavebands.

Red light is refracted by 42°, while at the other end of the spectrum, violet light is refracted by just 40°: hence our eyes perceive a band of light in different colours, and we'll only see a rainbow if that light hits our eyes at the exact angle.

This process or reflection and refraction is actually going on with regard to every ray of sunshine and every raindrop everywhere on Earth – it’s just that we only see a rainbow when light reaches our eyes from a particular set of raindrops.

And because the raindrops are spherical, the effect of this is that we see a rainbow as being curved.

Someone standing next to you will see a rainbow when you do, but it won’t be the same rainbow!

Oh, and one other thing… we tend to think of rainbows as being semi-circular, but they aren’t.

The processes described above are going on at every angle at once, so the rainbow they form is always a circle – it’s just that, as human beings, we’re usually at ground level, so half the rainbow that forms is normally hidden by the horizon.

In reality, all rainbows are full circles… maybe one day we’ll encounter an exoplanet where it rains liquid water in cubic or conical droplets, in which case that planet may experience square or triangular rainbows.

But if that ever happens, we’ll get back to you!