I am *pretty sure* it is because rainbow are a ‘refraction phenomenon’ – they happen when sunlight is bent (which means ‘refracted’) by raindrops. But you only see the effect at a certain angle, when the sun is behind you and you are looking at rain falling in the distance front of you. The light comes from the sun and is bent by the spherical raindrops and bounces back into your eye. If you trace a line from your eye to point in the rainstorm that keep the angle fixed, they describe a semi-circle.
That is a short answer and somewhat rubbishy answer I’m afraid but my eyes are closing since it’s nearly 11 O’clock at night!
Hi Maya – did you know that rainbows are actually circular? We see them as we do because one half is cut off from us due to the curvature of the Earth.
OK so I know you’re going to ask me now, why are rainbows circular? OK so i have googled this but it does make sense. You know how you can separate while light using a prism into all the colours of the rainbow? That’s what a raindrop does – separates all the different colours of light out, and each different colour is scattered at a slightly different angle. All the raindrops that are at a certain angle between your eye the sun form a circle in the sky – what a brilliant question!
I haven’t looked this up, but I think what happens is that raindrops (in front of you) bend light from the sun (behind you), and this bending (refraction) is bigger for some wavelengths of light than others. Light is only scattered back to your eye if the drop makes a particular fixed angle between the sun and you (slightly different for each different wavelength), and these drops lie on a circle in front of you (interrupted by the ground), which is why the rainbow is curved.
I hope that explanation is right, because I am sure some of the others will know the correct answer!
It is amazing to think that to be part of a rainbow, the light has to leave the sun (about 8 and half minutes earlier), travelling 93 million miles to rendezvous drops of water falling through the sky on a little patch of planet Earth, and then be twisted by precisely the right amount to pass through the pupil of your eye. Of course, if this coincidence hadn’t happened we wouldn’t have noticed anything unusual, but it is still mind-blowing, isn’t it?