Across the Electromagnetic Spectrum

Why do we use three different ways to describe the electromagnetic spectrum with three different physical measures? Because it is convenient – it is easier to understand 100 kilometers that one hundred thousand meters. So scientists normally use the most convenient measure for the spectral range they work with.

Thus, radio astronomers prefer frequencies or wavelengths. A large part of the radio domain is in the 1 cm to 1 km range, which corresponds to frequencies between 30 GHz and 300 kHz. This is a significant portion of the EM spectrum.

In the infrared (IR) and optical ranges, the basic measure is the wavelength. The IR-astronomy uses microns (short for micrometers, mcm or μm) for their range of wavelengths between 1 and 100 mcm. In the optical range, most convenient is the angstrom Å (1 Å = 10^–8 cm) or the nanometer (1 nm = 1 Å = 10^–7 cm). The optical range spans the 400 nm (blue-violet colour) to 700 nm (red) range. This is where our eyes are sensitive, but it covers only a small part of the whole EM spectrum.

The ultraviolet (UV), X-ray and gamma ray (γ-ray) wavelengths are very short. For these ranges the astronomers prefer to describe the photons using their energy, measured in electron volts (eV). UV-radiation spans the range from several eV to about 100 eV, X-rays go from 100 eV to 100 000 eV (or 100 keV), and γ-rays - above 100 keV.