<P> In the quantum theory of electromagnetism, EMR consists of photons, the elementary particles responsible for all electromagnetic interactions . Quantum effects provide additional sources of EMR, such as the transition of electrons to lower energy levels in an atom and black - body radiation . The energy of an individual photon is quantized and is greater for photons of higher frequency . This relationship is given by Planck's equation E = hν, where E is the energy per photon, ν is the frequency of the photon, and h is Planck's constant . A single gamma ray photon, for example, might carry ~ 100,000 times the energy of a single photon of visible light . </P> <P> The effects of EMR upon chemical compounds and biological organisms depend both upon the radiation's power and its frequency . EMR of visible or lower frequencies (i.e., visible light, infrared, microwaves, and radio waves) is called non-ionizing radiation, because its photons do not individually have enough energy to ionize atoms or molecules . The effects of these radiations on chemical systems and living tissue are caused primarily by heating effects from the combined energy transfer of many photons . In contrast, high ultraviolet, X-rays and gamma rays are called ionizing radiation since individual photons of such high frequency have enough energy to ionize molecules or break chemical bonds . These radiations have the ability to cause chemical reactions and damage living cells beyond that resulting from simple heating, and can be a health hazard . </P> <P> Maxwell derived a wave form of the electric and magnetic equations, thus uncovering the wave - like nature of electric and magnetic fields and their symmetry . Because the speed of EM waves predicted by the wave equation coincided with the measured speed of light, Maxwell concluded that light itself is an EM wave . Maxwell's equations were confirmed by Heinrich Hertz through experiments with radio waves . </P> <P> According to Maxwell's equations, a spatially varying electric field is always associated with a magnetic field that changes over time. Likewise, a spatially varying magnetic field is associated with specific changes over time in the electric field . In an electromagnetic wave, the changes in the electric field are always accompanied by a wave in the magnetic field in one direction, and vice versa . This relationship between the two occurs without either type field causing the other; rather, they occur together in the same way that time and space changes occur together and are interlinked in special relativity . In fact, magnetic fields may be viewed as relativistic distortions of electric fields, so the close relationship between space and time changes here is more than an analogy . Together, these fields form a propagating electromagnetic wave, which moves out into space and need never again affect the source . The distant EM field formed in this way by the acceleration of a charge carries energy with it that "radiates" away through space, hence the term . </P>

Who discovered that light is an electromagnetic wave