Ever seen a red rainbow?

24 December 2019

Rainbows happen when the white light from the Sun is refracted by raindrops in the air, a bit like the triangular prism on the front of the Dark Side of the Moon album.

But what happens if the sun is very close to setting, and the dust and atmosphere has blocked out all the blue end of the spectrum, giving the red colour of a sunset?

Well, this happens. The light that does hit the raindrops gets refracted, but there's no green or blue in the rainbow.

 

In telescopes and binoculars, this refraction is undesirable, and called chromatic aberration. Doublet and triplet refractor telescopes have specially designed lenses that counter this.