Electromagnetic Waves occur with frequencies ranging from 104 to 1024 Hz and have wavelengths from 10–16 to 104 meters. The common feature of all types of electromagnetic waves is that their speed in a vacuum is 3.00 × 108 m/s.
Radio waves make up the long wavelength, low-frequency end of the electromagnetic spectrum. The longest wavelengths (up to 10 km) are used for underwater communication (e.g., submarines). Long-wave radio stations use 1–2 km wavelengths (150–300 kHz) while medium-wave broadcasts use frequencies of up to 1.5 MHz (200 m wavelength). Short-wave radio frequencies continue up to 11–12 MHz (wavelengths down to a few meters). FM radio and TV transmissions use frequencies in the range of 60–900 MHz (wavelengths down to 300 cm).
Radar and Microwaves use the frequency range 1–20 GHz (wavelengths down to 1 cm).
Infrared radiation covers the wavelength region between 1 μ and 1 mm (frequencies between 1011 and ~3×1014 Hz).
Visible light occupies a small, but important, part of the electromagnetic spectrum. Frequencies between 4 and 8 × 1014 Hz and wavelengths decreasing from 7 to 4 × 10–7m, correspond to light with colors ranging from red, orange, yellow, green, blue and indigo light. Visible light, together with infrared and ultraviolet radiation, is emitted following electron transitions in atoms or molecules. Such transitions may be stimulated by heat or by the application of an electric current.
Ultraviolet rays have higher frequencies, from a few × 1016 Hz, and shorter wavelengths, down to 10–8 m, than visible light.
X-rays are emitted following electron transitions between the K, L, M and N atomic electron shells. X-ray wavelengths cover the range 10–8 to 10–11 m, with frequencies from 1016 to 1019 Hz. K X-rays are those produced by electron transitions to the K-shell; L X-rays are those produced by electron transitions to the L-shell and so on. Since these electron transitions involve the release of well-defined and precise quantities of energy, it is common to characterize X-rays by their energies and the normal units used are electron-volts. X-ray energies cover the range from a few eV to 100 keV.
Gamma rays are emitted by nuclear transitions and are usually emitted at higher energies than X-rays, with a range of a few keV to several MeV. Gamma rays have the shortest wavelength, down to 10–16 m, and the highest frequency, up to 1024 Hz, in the electromagnetic spectrum.
Characterizing X-rays and gamma rays by energy implies that these types of radiation can be thought of as small energy packets. These energy packets are called quanta or photons, and in some ways are particle-like. This dual nature of electromagnetic radiation extends right through the electromagnetic spectrum. Some features of electromagnetic radiation, such as interference or diffraction, are best explained with reference to electromagnetic waves whereas others, such as some X and gamma ray interactions are easier to envisage as particle-like interactions. Planck has shown that individual photons or quanta of electromagnetic radiation carry an energy proportional to frequency. The constant of proportionality is known as Planck’s Constant and has the value 6.63 × 10–34 J.s.