Why Is the Sky Blue?


Quick Answer

The sky appears blue because of the scattering of sunlight by atmospheric molecules, or Rayleigh scattering. Rayleigh scattering results in a blue sky because it is most noticeable at shorter wavelengths.

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Full Answer

The Earth's atmosphere contains a mixture of gases, as well as things like dust and drops of water. It is also constantly bombarded by light. Dust and water particles are much larger than a wavelength of light, meaning that any light hitting them bounces away. Gas molecules, on the other hand, are smaller than light waves. These molecules absorb light and re-emit it in different directions. Further, gas molecules have an easier time absorbing short wavelengths of light. So, most light on the red end of the spectrum passes through the atmosphere while gas molecules absorb the shorter waves and scatter them back as visible blue light.

A similar process occurs with larger particles and is less dependent on wavelength. This is called Mie scattering and tends to occur closer to the sun's location in the sky. Mie scattering results in the scattering of light of various wavelengths, which appears more like white light to the human eye and is the reason the sky appears paler closer to the sun.

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