十万个为什么·科学启蒙30篇（2）
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Why Is the Sky Blue?
天空为什么是蓝色的？
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Sunlight looks white, but it’s actually made of all visible colors — each with different wavelengths, from violet (shortest) to red (longest).
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When sunlight enters Earth’s atmosphere, gas molecules scatter shorter wavelengths like blue and violet far more than longer ones like orange and red.
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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.
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This scattering effect — called Rayleigh scattering — explains why the sky appears blue during daytime and reddish at sunrise or sunset.
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At dawn and dusk, sunlight passes through more atmosphere, so most blue light is scattered away before reaching your eyes — leaving warm tones behind.
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On the Moon, where there’s no atmosphere, the sky appears black even at noon because there’s nothing to scatter sunlight.
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Pilots flying above clouds sometimes see a deep indigo sky, as higher-altitude scattering includes more violet wavelengths.
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Pollution or dust particles cause Mie scattering, which scatters all wavelengths equally — turning skies hazy white or brown.