# Electromagnetic radiation/Units of measurenent

**Electromagnetic radiation: 2. Units of measurement**

Scientists usually use SI units. The unit of frequency is the hertz (Hz), one cycle per second, named after Heinrich Hertz. For our purposes, we shall use much higher frequencies. One thousand Hz is a kHz (kilohertz); one thousand kHz is a MHz (megahertz); one thousand MHz is a GHz (gigahertz); one thousand GHz is a THz (terahertz).

The SI unit of length is the metre (m). One thousand metres is a kilometre (km). One hundredth of a metre is a centimetre (cm); one thousandth is a millimetre (mm), one millionth is a micrometre (μm, often called a micron and written μ). A thousandth of a micrometre is called a nanometre (nm). A tenth of a nm (10^{−10} m) is known as an Angstrom unit or Angstrom (Å). The use of cm and Angstrom is discouraged in the SI system, because in that system multiples or submultiples of the basic unit (metres in this case) should only be used in powers of 1,000. However, many scientists like them.

The SI unit of energy is the Joule (J). However, there is a much smaller unit, the electron volt (eV). Again, its use is discouraged in the SI system, since its ratio to the Joule is not a power of 10. However, in some branches of physics it is a natural unit to use. The eV is the energy gained by an electron in passing through a potential difference of one volt. Since the charge on an electron is 1.60218 x 10^{−19} Coulombs, an eV is 1.60218 x 10^{−19} J. A keV is 1000 eV and a MeV is 1000 keV.

As will be discussed later, either frequency or wavelength may be used for radio waves. Wavelength is usually used for infrared, visible and ultraviolet (expressed in nm or Å), and photon energy for X-rays and gamma rays (expressed in keV or MeV).