十万个为什么·科学启蒙30篇（2）

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天空为什么是蓝色的？

Sunlight looks white, but it’s actually made of all visible colors — each with different wavelengths, from violet (shortest) to red (longest).
When sunlight enters Earth’s atmosphere, gas molecules scatter shorter wavelengths like blue and violet far more than longer ones like orange and red.
Although violet light scatters even more than blue, our eyes are less sensitive to violet, and some of it gets absorbed by the upper atmosphere.
This scattering effect — called Rayleigh scattering — explains why the sky appears blue during daytime and reddish at sunrise or sunset.
At dawn and dusk, sunlight passes through more atmosphere, so most blue light is scattered away before reaching your eyes — leaving warm tones behind.
On the Moon, where there’s no atmosphere, the sky appears black even at noon because there’s nothing to scatter sunlight.
Pilots flying above clouds sometimes see a deep indigo sky, as higher-altitude scattering includes more violet wavelengths.
Pollution or dust particles cause Mie scattering, which scatters all wavelengths equally — turning skies hazy white or brown.