Scattering of light by particles whose size is similar to the wavelength of light. It occurs in interstellar space and in the Earth’s atmosphere. For wavelengths much shorter than a given particle size, Mie scattering is a complex function of wavelength. Scattering drops to a minimum at a wavelength half the particle size, rises to a maximum when it is the same as the particle size, and then decays to zero towards longer wavelengths. Mie scattering can thus make objects appear either redder or bluer according to the size of the scattering particle. In the Earth’s atmosphere there is usually a wide range of particle sizes, so that these colour effects are blurred into a nondescript grey. Very occasionally, atmospheric effects will combine so that the dust is mostly 900 nm in size and the minimum in scattering is at 450 nm. This is the wavelength of blue light that causes the phenomenon of the blue Moon (1). Mie scattering is named after the German physicist Gustav Adolf Feodor Wilhelm Ludwig Mie (1868–1957).