When a double rainbow happens there’s a dark shadow between the bows. This is Alexander’s band.
Last week Paul “Bear” Vasquez, the star of the “double rainbow” viral video, died. That’s a bummer; in the video he was just so beautifully enthusiastic about those double rainbows, and we all need a little more of that kind of light in our lives.
The way that rainbows are formed is pretty well known: sunlight bounces through raindrops and separates into different wavelengths, giving us the characteristic splay of colours. But some light is just refracted, lightening the sky directly inside the rainbow’s curve. The sky outside the rainbow’s curve doesn’t get any of that refracted light, so it’s comparatively dark.
This is especially noticeable when you see a double rainbow. The light from the first rainbow is refracted inwards, but the light from the second rainbow is refracted outwards instead. (The colours of the outer rainbow are also reversed.)
All this leaves a large shadow between the two rainbows. It’s called Alexander’s band, after the 200 CE Greek philosopher named Alexander of Aphrodisias who first described it. He also argued against the immortality of the soul, which I suspect does not agree with Vasquez’s view of the universe.