Fun with Units

The total range of light wavelengths we can measure spans an incredible range of almost 20 powers of ten from the shortest gamma rays to the longest radio wavelengths. The devices needed to measure these different kinds of light are very different from one another and were developed at different times by differing groups of scientists. It is perhaps not too surprising then that the conventions developed in each wavelength range, such as the units used to describe the wavelengths or other such nomenclature, vary from one spectral region to another.

Letting our personal biases show, it may be helpful to again refer things to the optical region. Optical wavelengths are very tiny, and a unit called the Angstrom is often used. One Angstrom is defined to be 10^{-10} meters, and visible light spans the range from about 4000 to 7000 Angstroms (a little less than a factor of two in relative wavelengths). For comparison, the near-ultraviolet is usually defined as the region from 2000 Angstroms up to the visible, the far-ultraviolet goes down to 912 Angstroms, and the extreme-ultraviolet extends down to about 50 Angstroms. Hence, ultraviolet light extends over a factor of about 80 in relative wavelengths! (I told you the optical range was just a tiny part of the entire spectrum!)

At wavelengths longer than optical light, the Angstrom unit becomes increasingly awkward. At 10,000 Angstroms, astronomers usually convert over to another unit called a micron, which is one-millionth of a meter. (Hence, 10,000 Angstroms = 1 micron.) This seems to be the unit of choice for the infrared. For instance, the Infrared Astronomy Satellite, known as IRAS, observed the entire sky in the early 1980's at four infrared wavelengths: 12, 25, 60, and 100 microns. (Note again--infrared region extends over more than a factor of 100 in relative wavelengths!)

In the radio part of the spectrum, you could measure the wavelengths with a ruler or a yardstick--if you could see radio light! (And if you could get a light wave to stand still!) The shortest wavelengths usually ascribed to the radio region are about a millimeter and the longest extend up to tens or even hundreds of meters, more than a factor of 100,000 in relative wavelength!

For historical reasons, radio astronomers often refer to the frequency of radio light instead of the wavelength. The frequency is just the number of wavelengths per second passing some reference point: the shorter the wavelength the more of them that can pass the reference point per second (that is, the larger the frequency). The unit of 1 wavelength per second is called a Hertz (Hz). LOTS of wavelengths per second can pass a reference point, and radio waves are measured in units of thousands (kiloHertz or KHz, like AM radio), millions (megaHertz or MHz, like FM radio), or billions (gigaHertz or GHz) of wavelengths per second. Most wavelengths used in astronomy are in the MHz and GHz range.

For the X-ray and gamma ray regions, the Angstrom unit is again abandoned because the wavelengths become much less than a single Angstrom! In these regions, it is conventional to show the spectra on a scale in energy units instead of wavelengths. (Of course these two are related directly, so it is just the units that are changing.) The energy unit of choice is called the electron volt (or eV), with the same prefixes as used with the Hertz to indicate larger and larger energies. X-rays range from about 0.1 - 100 kilo-electron volts (KeV), while gamma rays extend all the way up to giga-electron volt (GeV) energies. (For comparison, an optical photon would have an energy expressed as only about 2 electron Volts, or about one 500-millionth of a typical gamma ray!)

Even though the units of measurement are so different from one region to the next, it is important to realize that they are all measuring the same thing! Light is just light, no matter what part of the EM spectrum is being discussed, and it is just the characteristic length of the electromagnetic vibration (or the energy carried by each light photon) that is changing across the EM spectrum!

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Bill Blair (