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In the passage of a beam of light through a medium, some of the radiant energy is absorbed and is transformed into internal energy, while some of it is scattered in all directions. The oscillating electric field associated with the light wave sets some of the electrons of the medium into oscillation, thus giving up some of its energy, and these oscillating electrons subsequently reradiate energy as scattered electromagnetic radiation. Scattering therefore takes place only in the presence of matter. The color of the sky is due to the small amount of scattering of sunlight by the molecules of the air. At high altitudes the number of scattering particles diminishes, and the sky is darker. In the stratosphere the sky appears almost black. The molecules of the air are more effective in scattering short wavelengths, so that the scattered light appears blue. Since the shorter wavelengths are scattered from the direct beam, the beam appears redder as it passes through larger thicknesses of air. It is for this reason that the setting sun looks redder than the noonday sun. The scattering of light by the air is responsible for twilight. In microwave communications the distance between transmitting and receiving antennae is limited by the curvature of the earth, because of the straight-line propagation of electromagnetic waves. By transmitting a stronger initial signal and detecting the scattered microwaves, it has been possible to reduce the number of transmitting and receiving stations in the microwave communications network by increasing the distance between relay stations beyond the line of sight.