Dimming of light from the stars due to absorption and scattering by grains of dust in the interstellar medium. In the absorption process the radiation disappears and is converted into heat energy in the interstellar dust grains. In the scattering process the direction of the radiation is altered. Interstellar extinction produces a dimming of the light from stars situated beyond interstellar clouds of dust according to the equation F_λ = F_λ(0)e^−τλ, where F_λ is the observed flux of radiation from the star at a wavelength λ, F_λ(0) the flux that would be observed in the absence of interstellar extinction, and τ_λ the dimensionless optical depth for interstellar extinction at λ. Measures of the radiation from pairs of stars of similar intrinsic properties but with differing amounts of interstellar extinction can be used to obtain information about τ_λ, which can then be used to provide clues about the nature of the interstellar dust grains.