The visible region is only a minute part of the entire electromagnetic spectrum, which is shown at the left. Because of the vast range of wavelengths involved, a logarithmic, or power-of-ten, scale has been used instead of a linear scale. Each successive division corresponds to a tenfold increase in wavelength. At wavelengths shorter than violet, the ultraviolet continues to approximately 100, where an ill-defined border separates far ultraviolet from x-rays. Radiation at atomic dimensions, around 1 , is called x radiation, but when these and finer waves are generated in atomic reactions they are called gamma () rays. At longer wavelengths, the far infrared gives way to microwaves and then to radio waves, of which several of the more familiar types are shown on the spectrum. The shortest gamma rays represented on the spectrum are only a few hundredths the size of an atom, and the longest radio waves shown are seven miles long.