十万个为什么·科学启蒙30篇（1）
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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 actually contains all colors of the rainbow mixed together.
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When sunlight enters Earth’s atmosphere, tiny air molecules scatter the light in all directions.
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Blue light has shorter wavelengths and scatters much more easily than red or yellow light.
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This strong scattering makes blue light reach our eyes from many different angles across the sky.
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At sunrise or sunset, sunlight travels through more atmosphere, so most blue light is scattered away.
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The remaining light that reaches us directly is richer in red and orange wavelengths instead.
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That’s why the sky near the sun appears golden or fiery during those times.
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Without an atmosphere—like on the Moon—the sky would always look black, even at noon.
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Scientists use this same scattering principle to study the composition of distant planetary atmospheres.
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You can even see a hint of this effect by shining a flashlight through a glass of water with a drop of milk.