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If you know the explanation for why the sky's blue,
如果你知道天空为什么是蓝色的，
you've heard that sunlight is white light composed of many frequencies
也听说过阳光是由很多频率组合成的白光
and that higher frequency, bluer, light scatters in the air more than lower frequency, redder, light.
且较高频率、较蓝的光比起较低频率、较红的光更容易在空气中散射。
So, you might ask 'why isn't the sky violet, since violet is an even higher frequency than blue?'
所以你可能会问‘为什么天空不是紫色的，紫色比蓝色频率更高啊？’
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…
首先，紫外线和X光频率也比蓝光高，这并不意味着天空是X光色...
Both 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 -
因为我们看不见它们，也是因为太阳发出的X光也不多，而且会被大气阻挡—
which is probably why we didn't evolve to be able to see them.
这可能是因为我们没有进化出看见它们的能力。
And secondly, the color violet in the rainbow is the 'roses are red, violets are BLUE' violet.
第二，彩虹中的紫色是‘玫瑰红、紫罗兰’的紫色。
Not purple – it's dark blue. To see why, check out this Chromaticity Diagram!
不是一般的紫色，是接近于深蓝色的。看看这张色度图就能知道原因！
It's a graph of all possible colors as perceived by non-colorblind humans, ignoring brightness and context.
这张图上是所有人类能看到的所有颜色，忽略亮度和背景色。
The color displayed on the diagram is just to give you a rough idea –
这张图上的颜色只是给你一个概念——
unfortunately your computer can't display all possible perceived colors –
但你的电脑无法展现出所有这些颜色——
it can just display the ones inside a triangle like this, and is pretending for the rest.
电脑只能展现出这个三角形内的颜色，其他的不能呈现。
Single frequency light like that from a laser or from splitting white light into a rainbow is graphed around this outside edge,
激光发出的单频光或者白光分裂成彩虹发出的单频光在这个外边缘被画成图，
while any color anywhere on the inside or along the bottom edge can only be made with a combination of various frequencies of light.
而内部或底部边缘任何一处的颜色只能通过各种光的频率的组合才能得到。
As you can see, pink, purple and magenta can only be made using multiple frequencies of light -
就像大家所能看的的一样，红色、紫色和洋红色只能用不同频率色光混合产生——
there is no single frequency of light that's pink - and of course, white light is a mixture as well!
没有单独粉色光的频率——当然，白光也是混合所产生的的。
Which is why you don't see any of these colors in rainbows made from prisms.
所以你在三棱镜射出的彩虹中看不见其中任何一种颜色。
But back to the Chromaticity diagram! See this line in the middle?
但是回到色度图！看到中间这条线了么？
This shows what color hot objects glow - we have red hot, then white hot, then blue hot.
这表明炽热物体所发出的颜色——我们有发红、发白和发蓝
All of these 'hot' colors are made up of a broad range of frequencies, since that's what hot objects emit.
所有这些颜色都是由各种广泛频率所形成的，因为这些物体可以那些光。
The sun, for example, is white hot before it hits the atmosphere,
例如，太阳在照到大气前是白色的，
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.
然后它的光被空气分子散射开来，所以太阳看起来有点红，其余的天空看起来有点蓝。
Like this picture of a sunset! And the thing is this - if you notice the line for hot-object color,
就像这幅日落图！重点是——如果你仔细看炽热物体发出的色光曲线，
well, it stops in the middle of the diagram near whitish-blue. It never goes beyond that.
它停在这幅图的中间，接近白色-蓝色。 它永远不会超出这个范围。
And that's because the spectrum of light from an object hotter than the sun always has a decreasing tail,
这是因为来自比太阳热的物体的光谱总是有一个逐渐减小的残留，
with slightly more blue than green than red, so you never get the right combination of frequencies to push the color towards purple,
蓝色比绿色多，绿色比红色多，所以你永远制造不出紫色所需的频率的结合，
or for that matter, even towards a pure, deep violet blue.
就这一点而言，即使是纯的、深紫色的蓝色也不行。
Kind of like how you can mix flour and water in different ratios to make bread, pancakes,
就像是把面粉和水用不同比例混合，能做出面包、煎饼
or vegetarian gravy, but as long as you have flour in there, your gravy won't be gluten free.
或素食肉汁，但只要你有面粉在里边，你的肉汁就不可能不含麸质。
And that's why the sky appears blueish white. It's basically gravy.
这就是天空呈蓝白色的原因。基本上那就是些肉汁。
