" When light is scattered by particles which are very small compared with any of the wavelengths, the ratio of the amplitudes of the vibrations of the scattered and incident light varies inversely as the square of the wavelength and the intensity of the lights themselves as the inverse fourth power."

J.W. Strutt (Lord Rayleigh), 1871

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Small airborne particles (aerosols) are responsible for a variety of well known optical phenomena in the atmosphere, e.g. rainbows, halos and deep red-coloured sunrises and sunsets. The latter is a direct consequence of selective extinction of sunlight by Rayleigh- and Mie-scattering. Light scattering and absorption by anthropogenic aerosol emissions from biomass-burning and fuel-combustion may affect the atmospheric heat budged, i.e. the earth's climate, the visibility, and even photochemical reaction rates. Due to their high spatial and temporal variability the influences of aerosols on climate are much more complex than those for greenhouse gases. Different sources and production mechanisms provide a wide variety of different compositions and morphologies. Moreover, during the atmospheric residence times, from less than a day to more than a month, the microphysical aerosol properties are aged. A detailed knowledge of aerosol optical properties  is required to assess the direct shortwave radiative forcing of anthropogenic aerosols.
Similar to aerosols in the earth's atmosphere small particles embedded in dense interstellar molecular clouds are important for the heat budged of these clouds. Interstellar dust plays a crucial role in the process of star formation by effectively cooling the collapsing molecular cloud. In the diffuse interstellar medium the absorption and scattering characteristics of the cosmic dust are reflected by the shape of the interstellar extinction curve. Here, also a detailed knowledge the optics of small particles is necessary to understand the physics of interstellar clouds and to deduce information about the nature of the solid material in interstellar space.