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.