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  • 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-raysBoth because we can't see them and also because the sun doesn't make

  • very many of them and then the atmosphere blocks them all - which is probably why we

  • didn't evolve to be able to see them. And secondly, the color violet in the rainbow

  • is the "roses are red, violets are BLUE" violet. Not purpleit's dark blue.

  • To see why, 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

  • is just to give you a rough ideaunfortunately your computer can't display all possible perceived

  • colorsit 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 from splitting white light into a rainbow

  • is graphed around this outside edge, while 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! Which 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 color

  • hot objects glow - we have red hot, then white hot, then blue hot. All of these "hot" colors

  • are made up of a broad range of frequencies, since that's what hot objects emit. The sun,

  • for example, 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 kind

  • of 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 more blue 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 basically gravy.

If you know the explanation for why the sky's blue, you've heard that sunlight is white


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B1 中級 美國腔

天空為什麼不是紫色的? (Why Isn't The Sky Purple?)

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    Wayne Lin 發佈於 2021 年 01 月 14 日