Why is the sky blue?

The daytime sky appears blue because molecules in Earth's atmosphere scatter shorter wavelengths of sunlight, like blue light, much more intensely than longer wavelengths, like red light. This optical phenomenon, known as Rayleigh scattering, happens when light interacts with particles that are much smaller than the light's wavelength. As the scattered blue light radiates in all directions, it creates the diffuse blue sky we see overhead.

What we know

• Rayleigh scattering is the elastic scattering of light or other electromagnetic radiation by particles whose size is much smaller than the radiation's wavelength. [1, 2]
• The amount of scattering is inversely proportional to the fourth power of the wavelength, meaning blue light scatters much more than red light. [2]
• Rayleigh scattering arises from the electric polarizability of particles, where an oscillating light wave causes charges within a particle to move and radiate as a dipole. [2]
• Daytime diffuse sky radiation is caused by the Rayleigh scattering of sunlight in Earth's atmosphere. [2, 3, 4]

Where to go next

• whenWhat happens to light during twilight?
• compared withHow does Mie scattering differ from Rayleigh scattering?
• whoWho was Lord Rayleigh?
• the bigger pictureWhat is diffuse sky radiation?
• a case in pointWhy are human eyes blue?
• an unexpected connectionHow does scattered sky light help plants grow?Diffused skylight resulting from atmospheric scattering can penetrate and illuminate under-canopy leaves, allowing for more efficient whole-plant photosynthesis than direct sunlight alone.

Sources

• [1]Rayleigh scattering (Wikidata Q193709) · CC0 1.0
• [2]Rayleigh scattering · CC BY-SA 4.0
• [3]Diffuse sky radiation · CC BY-SA 4.0
• [4]Sky · CC BY-SA 4.0
• [5]Blue · CC BY-SA 4.0