The atmosphere of the sun is transparent to visible radiation, is nearly transparent to millimetre and centimetre radio radiation, and becomes opaque to the metre and longer wave radiation. Information about the chromosphere can then be given by observing the radiation from the sun at short radio wave-lengths. In its outer part, the atmosphere of the sun is highly ionized. Absorption in any region is directly proportional to the square of the density and the wave-length squared and inversely to the temperature to the three-halves power This is the familiar equation for the absorption of radio waves in an ionized medium. By consequence of this, the longer wave radiation is absorbed in the outer layers of the sun's atmosphere and can escape only from these outer regions. The shorter wave-length radiation is absorbed very little in the outer part of the solar atmosphere where the density is quite low, and hence radiation from the chromosphere escapes as centimetre and millimetre radio waves. In fact, the principal radiation from the sun in the centimetre and millimetre region comes from the chromosphere.