Radiation

Radiation is the flow through space of atomic and subatomic particles, such as electrons (β-radiation), helium nuclei (α-radiation) and high-energy photons (γ-radiation). The moving particles carry energy and accordingly radiation can be described as the transmission of energy through space. It is one of the important principles of quantum mechanics that beams of (sub)atomic particles can be seen as consisting of waves, see wave-particle duality, and therefore radiation can also be defined as the transport of energy through space by means of waves.

An important form of radiation is electromagnetic radiation, which in the 19th century was seen as a wave phenomenon, but, as was proposed by Einstein in 1905, can also be seen as consisting of photons—massless particles that propagate through space with the speed of light. In fact, light is a form electromagnetic radiation. Electromagnetic radiation covers a large spectrum, from low-energy radio waves to highly energetic X-rays and γ-rays. Visible light is somewhere in the middle of this spectrum.

High-energy radiation, such as X-rays and γ-rays, can ionize matter, i.e., remove electrons from the atoms that constitute matter. For living cells such ionization can be damaging, see ionizing radiation. Certain atomic nuclei are radioactive, that is, they emit α, β, or γ radiation (or mixtures of the three kinds of radiation); often this nuclear radiation is highly energetic, causing injury to living beings.