Astronauts orbiting Earth see a remarkable ‘thin blue line’, where the Sun is close to the horizon (Figure 1). This is explained by a process known as Rayleigh scattering (Figure 2).
When electromagnetic radiation encounters particles much smaller than its own wavelength, some of the radiation is scattered. This Rayleigh scattering is elastic — there is no change in the photon energy, and no change in colour. The wavelength, λ, of visible light ranges from about 400nm to 700nm. In any given direction, the intensity, I, of scattered radiation is given by:
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