<P> In the Liénard--Wiechert potential formulation of the electric and magnetic fields due to motion of a single particle (according to Maxwell's equations), the terms associated with acceleration of the particle are those that are responsible for the part of the field that is regarded as electromagnetic radiation . By contrast, the term associated with the changing static electric field of the particle and the magnetic term that results from the particle's uniform velocity, are both associated with the electromagnetic near - field, and do not comprise EM radiation . </P> <P> Electrodynamics is the physics of electromagnetic radiation, and electromagnetism is the physical phenomenon associated with the theory of electrodynamics . Electric and magnetic fields obey the properties of superposition . Thus, a field due to any particular particle or time - varying electric or magnetic field contributes to the fields present in the same space due to other causes . Further, as they are vector fields, all magnetic and electric field vectors add together according to vector addition . For example, in optics two or more coherent lightwaves may interact and by constructive or destructive interference yield a resultant irradiance deviating from the sum of the component irradiances of the individual lightwaves . </P> <P> Since light is an oscillation it is not affected by traveling through static electric or magnetic fields in a linear medium such as a vacuum . However, in nonlinear media, such as some crystals, interactions can occur between light and static electric and magnetic fields--these interactions include the Faraday effect and the Kerr effect . </P> <P> In refraction, a wave crossing from one medium to another of different density alters its speed and direction upon entering the new medium . The ratio of the refractive indices of the media determines the degree of refraction, and is summarized by Snell's law . Light of composite wavelengths (natural sunlight) disperses into a visible spectrum passing through a prism, because of the wavelength - dependent refractive index of the prism material (dispersion); that is, each component wave within the composite light is bent a different amount . </P>

Where does the energy for the emr originate