Electromagnetic radiation (EMR) consists of self-propagating electromagnetic waves (such as light), which can travel through a vacuum, and sometimes matter as well. EMR has electrical and magnetic field components, which oscillate in phase perpendicular to each other, and to the direction of propagation of the wave. EMR behaves as both wave and particle; the particles are photons. As waves, EMR has both frequency and wavelength.

The electromagnetic spectrum is simply the range of all electromagnetic frequencies, usually displayed as a chart. Frequencies can be grouped by the use we put them to, such as radio waves, visible light, x-rays, microwaves, etc.

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Before getting into a description of the spectrum, here’s a collection of ancillary information that might be useful.

Electron Volt

This is a unit of energy in physics. It’s defined as the energy gained by a single unbound electron, as it accelerates through a potential difference of 1 volt. The charge of an electron is 1.602176487×10-19 volts. Therefore:

1 eV = 1.602176487×10-19 J

Since energy can be converted to mass, we can also think of an eV as a unit of mass. An atomic mass unit (amu, or simply u) is a small unit used in measuring mass at the atomic level. An amu is approximately the mass of a hydrogen atom, or a proton, or neutron (approximately, not exactly). It is defined as 1/12 the mass of an unbound 12C atom at rest, in its ground state. To convert it into grams, divide by Avogadro’s Number (NA):


1 amu = 1/NA grams = 1/6.02214179×1023 * 1/1000 kg = 1.660538782×10-27 kg


Since e = mc2, we can calculate the mass equivalence:

1 eV = 1.602176487×10-19 / c2 = 1.602176487×10-19 / 2997924582 = 1.78266×10-36 kg

1 amu = 1.660538782×10-27 / 1.78266×10-36 eV = 9.31494×108 eV = 0.93149 GeV


A proton is slightly heavier than an amu, at approximately 0.938 GeV, making the GeV a particularly handy unit for physicists. These energy scales are also useful in relating the energy of EMR to various other things, as can be seen in the table.

The energy of EMR can be calculated with the formula E = hν or E = hc/λ where h is Planck’s Constant, c is the speed of light in vacuum, ν is frequency of the EMR and λ is the wavelength of the EMR. Planck’s constant has a value of 6.62606885×10-34 J.s, or 4.13566733×10-15 eV.s depending on units used.

Since h and c are constants, we can express the formula E = hc/λ as:

E = hc/λ = (6.62606885×10-34 * 299792458) / λ = 1.98645×10-25 / λ J, or (4.13566733×10-15 * 299792458) / λ

= 1.23984×10-6 / λ eV

This last formulation is especially interesting, since in order to make it unity, we could simply select a λ equal to 1.23984×10-6 which means that EMR with a wavelength of 1.23984×10-6 meters (about 1240 nm) has an energy of 1 eV per photon. This would be within the infra-red part of the spectrum.

Highest energy Cosmic Rays (protons) 3×1020 eV about the energy of a baseball pitched at 60 miles per hour
Medical radiotherapy X-Rays 20 MeV  
Medical diagnostic X-Rays 15-30 keV these are hard X-Rays
Visible Light 1.7 – 3 eV this is the part of the spectrum we see

The Electromagnetic Spectrum

The image above shows the range of the electromagnetic spectrum and some common uses for different parts of the spectrum. The scale on the left is frequency, and the scale on the right is the wavelength.