Electromagnetic radiation can be described using energy, wavelength, or frequency. Frequency is measured in cycles per second or Hertz, wavelength in meters, and energy in electron volts. While these three quantities are related through precise mathematical relationships, why have three ways of describing things, each with a different set of physical units?
In brief, scientists prefer to use units that are convenient and avoid excessive numerical values. For example, it is simpler to express “two kilometres” rather than “two thousand meters.” Typically, scientists utilize the units that are most convenient for the specific type of electromagnetic radiation they are studying.
Astronomers specialising in radio wave research primarily employ measurements of wavelengths or frequencies. The radio portion of the electromagnetic spectrum typically spans from approximately 1 cm to 1 km, corresponding to frequencies ranging from 30 gigahertz (GHz) to 300 kilohertz (kHz). The radio spectrum encompasses a wide range within the electromagnetic spectrum.
Infrared and optical astronomers primarily utilise measurements of wavelength. For infrared astronomers, wavelengths are expressed in microns (millionths of a meter), typically ranging from 1 to 100 microns within the electromagnetic spectrum. On the other hand, optical astronomers employ both angstroms (0.00000001 cm, or 10-8 cm) and nanometres (0.0000001 cm, or 10-7 cm). Using nanometres, violet, blue, green, yellow, orange, and red light have wavelengths between 400 and 700 nanometres. It’s important to note that this range represents only a small portion of the entire electromagnetic spectrum, meaning the light we can see constitutes a tiny portion of the overall electromagnetic radiation present in our surroundings.
In the ultraviolet, X-ray, and gamma-ray regions of the electromagnetic spectrum, the wavelengths are extremely short. Astronomers specialising in these regions typically describe the photons in terms of their energies, measured in electron volts (eV), instead of wavelengths. Ultraviolet radiation spans from a few electron volts up to approximately 100 eV, while X-ray photons have energies ranging from 100 eV to 100,000 eV (or 100 keV). Gamma rays encompass all photons with energies exceeding 100 keV.
Reference: NASA