Electromagnetic radiation is pure energy propagating through space or substances. This energy has no mass and no physical form, but we can measure its presence by using sensors that absorb the radiation.

Because there is no physical form to the energy, the radiation is not going to look like the sine wave in the diagram. However, the electromagnetic radiation propagates in a way that obeys the laws of wave propagation, so we can characterize the radiation as having some wavelength. At the same time, the radiation propagates in a way that obeys the laws of mechanics for discrete objects (ones that have no mass, that is), so we can say that the radiation propagates as separate blobs of energy called photons.

The longer the wavelength of the radiation, the less energy there is in a single photon of this radiation. Short wavelength radiation has a higher potential for damaging living tissue because of the high amount of energy per photon.

The types of electromagnetic radiation shown here are the most important ones for radiation energy propagating through the Earth System.