There was a magnificent rainbow in Surrey on Friday. It was late afternoon, the Sun was low and the sunshine caught a shower of rain against a backdrop of dark clouds. The colours of the rainbow were extraordinarily vivid violet on the inside and red on the outside. But another faint bow appeared underneath the main rainbow, with indistinct bands of green, pink and purple. This was a supernumerary arc and is not often seen.

Supernumeraries become visible when the rain droplets are nearly all the same size. They are usually clearest near the top of a rainbow. For years they remained an enigma, long after scientists had established that rainbows were created by light refraction inside raindrops. Rainbows fitted Isaac Newton’s theory that light was made up of a series of small particles, but it could not explain supernumeraries. However, in 1803 the English scientist Thomas Young claimed that supernumeraries could be explained if light behaved as waves, not particles. As the waves of light reflected inside raindrops they crashed into incoming light rather like ripples bouncing off the sides of the bath to create an interference pattern. A similar phenomenon with light in raindrops produced extra bands of light.

Young faced scathing attacks for his theory and was accused of disrespecting Newton. But the wave theory of light won out and dominated in the 19th century. Today we view light as both waves and particles.

Apologies for an error in yesterday’s Weather Eye: the official hurricane season ends on November 30, not November 1.