Why Isn’t The Sky Purple?
If you know the explanation for why the sky’s blue, you’ve heard that sunlight is white light composed of many frequencies and that higher frequency, bluer, light scatters in the air more than lower frequency, redder, light.
So, you might ask, “why isn’t the sky violet, since violet is an even higher frequency than blue?”. First off, ultraviolet light and x-rays are also higher frequency than blue, and that doesn’t mean the sky is the color of x-rays..Both because we can’t see them and because the sun doesn’t make very many of them. Then the atmosphere blocks them all – which is probably why we didn’t evolve to see them.
And secondly, the color violet in the rainbow is the “roses are red, violets are BLUE” violet. Not purple – it’s dark blue. To see why to check out this Chromaticity Diagram! It’s a graph of all possible colors as perceived by non-colorblind humans, ignoring brightness and context. The color displayed on the diagram gives you a rough idea – unfortunately, your computer can’t display all possible perceived colors – it can just display the ones inside a triangle like this and is pretending for the rest.
Single-frequency light like that from a laser or splitting white light into a rainbow is graphed around this outside edge. In contrast, any color anywhere on the inside or along the bottom edge can only be made with a combination of various frequencies of light. As you can see, pink, purple, and magenta can only be made using multiple frequencies of light – there is no single frequency of light that’s pink – and of course, white light is a mixture as well! This is why you don’t see any of these colors in rainbows made from prisms. But back to the Chromaticity diagram! See this line in the middle? This shows what hot color objects glow – we have red hot, then white-hot, then blue hot. These “hot” colors are made up of a broad range of frequencies since they emit hot objects. For example, the sun is white-hot before it hits the atmosphere, then its light is split by scattering off of air molecules, so the sun looks slightly red hot, and the rest of the sky looks blue hot.
Like this picture of a sunset! And the thing is this – if you notice the line for hot-object color, well, it stops
in the middle of the diagram near whitish-blue. It never goes beyond that. And that’s because the spectrum of light from an object hotter than the sun always has a decreasing tail, with slightly bluer than green than red, so you never get the right combination of frequencies to push the color towards purple, or for that matter, even towards a pure, deep violet-blue.
Kind of like how you can mix flour and water in different ratios to make bread, pancakes, or vegetarian gravy, but as long as you have flour in there, your gravy won’t be gluten-free. And that’s why the sky appears blueish white. It’s gravy.
