The apparent reddening of light from distant stars by interstellar dust, which causes scattering. The extent to which light is scattered and absorbed in the interstellar medium depends strongly on wavelength: blue light is dimmed more than red light. As a result, the colours of stars viewed through interstellar material are altered and appear redder. The degree of reddening increases with the quantity of intervening matter. A similar effect in the Earth’s atmosphere causes the reddening of the Sun when it is close to the horizon.