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Electromagnetic radiation is a self propagating Wave in space with Electric and Magnetic components. These components oscillate at right angles to each other and to the direction of propagation. The term ''electromagnetic radiation'' is also used as a synonym for electromagnetic waves in general, even if they are not radiating or travelling in free space. This sense includes, for example, Light travelling through an Optical Fiber , or Electrical Energy travelling within a Coaxial Cable . Electromagnetic (EM) radiation carries Energy and Momentum which may be imparted when it interacts with Matter . PHYSICS Theory Electromagnetic waves of much lower frequency than visible light were predicted by Maxwell's Equations and subsequently discovered by Heinrich Hertz . Maxwell Derived a wave form of the electric and magnetic equations which made explicit the wave nature of the electric and magnetic fields. These equations displayed the symmetry of the fields. According to the theory, a time-varying Electric Field generates a Magnetic Field and vice versa. Thus, an oscillating electric field creates an oscillating magnetic field, which in turn creates an oscillating electric field, and so on. By this means an EM wave is produced which propagates through space. Properties Electric and magnetic fields exhibit the property of Superposition . This means that the field due to a particular particle or time-varying electric or magnetic field adds to the fields due to other causes. (As magnetic and electric fields are vector fields, this is the Vector addition of all the individual electric and magnetic field vectors.) As a result, EM radiation is influenced by various phenomena such as Refraction and Diffraction . For example, a travelling EM wave incident on a particular arrangement of Atom s induces oscillation in the atoms and thus causes them to emit their own EM waves. These emissions interfere with the impinging wave and alter its form. In refraction, a wave moving from one medium to another of a different density changes its speed and direction when it enters the new medium. The ratio of the refractive indices of the media determines the extent of refraction and is summarized by Snell's Law . Refraction is the mechanism by which light disperses into a Spectrum when it is shone through a prism. The Physics of electromagnetic radiation is Electrodynamics , a subfield of Electromagnetism . EM radiation exhibits both wave properties and particle properties at the same time (see Wave-particle Duality ). These characteristics are mutually exclusive and appear separately in different circumstances: the wave characteristics appear when EM radiation is measured over relatively large timescales and over large distances, and the particle characteristics are evident when measuring small distances and timescales. These characteristics have been confirmed by a large number of experiments. Wave model An important aspect of the wave nature of light is Frequency . The frequency of a wave is its rate of oscillation and is measured in Hertz , the SI unit of frequency, equal to one oscillation per Second . Light usually comprises a spectrum of frequencies which sum to form the resultant wave. In addition, frequency affects properties like Refraction , in which different frequencies undergo a different level of refraction. A wave has troughs and crests. The Wavelength is the distance from crest to crest. Waves in the electromagnetic spectrum vary in size from very long radio waves the size of buildings, to very short gamma-rays smaller than the size of the nucleus of an atom. Frequency has an inverse relationship to the concept of wavelength. When waves travel from one medium to another, their frequency remains exactly the same - only their speed changes. Waves can also be described by their Radiant Energy . Interference is the superposition of two or more waves resulting in a new wave pattern. The way that these coincide causes different types of interference. Particle model In the particle model of EM radiation, EM radiation is Quantized as particles called Photon s. Quantisation of light represents the discrete packets of energy which constitute the radiation. The frequency of the radiation determines the magnitude of the energy of the particles. Moreover, these particles are emitted and absorbed by charged particles, so photons act as transporters of Energy . A photon absorbed by an Atom excites an Electron and elevates it to a higher Energy Level . If the energy is great enough, so that the electron "jumps" to a high enough energy level, it may escape the positive pull of the nucleus and get liberated from the atom in a process called Ionization . Conversely, an electron which descends to a lower energy level in an atom emits a photon of light equal to the energy difference. The energy levels of electrons in atoms are discrete. Therefore, each element has its own characteristic frequencies. Together these effects explain the absorption spectra of Light . The dark bands in the spectrum are due to the atoms in the intervening medium which absorb different frequencies of the light. The composition of the medium through which the light travels determines the nature of the absorption spectrum. For instance, in a distant star, dark bands in the light it emits are due to the atoms in the atmosphere of the star. These bands correspond to the allowed energy levels in the atoms. A similar phenomenon occurs for emission. As the electrons descend to lower energy levels, a spectrum which represents the jumps between the energy levels of the electrons is exhibited. This is manifested in the emission spectrum of Nebula e. Today, scientists use this phenomenon to observe what elements a certain star is composed of. It is also used in the determination of the distance of a given star, using the so-called Red Shift . Speed of propagation Any electric charge which accelerates, or any changing magnetic field, produces electromagnetic radiation. Electromagnetic information about the charge travels at the speed of light. Accurate treatment thus incorporates a concept known as Retarded Time (as opposed to advanced time, which is unphysical in light of Causality ), which adds to the expressions for the electrodynamic Electric Field and Magnetic Field . These extra terms are responsible for electromagnetic radiation. When any wire (or other conducting object such as an Antenna ) conducts Alternating Current , electromagnetic radiation is propagated at the same frequency as the electric current. Depending on the circumstances, it may behave as a Wave or as Particle s. As a wave, it is characterized by a velocity (the Speed Of Light ), Wavelength , and Frequency . When considered as particles, they are known as Photon s, and each has an energy related to the frequency of the wave given by Planck's relation ''E = hν'', where ''E'' is the energy of the photon, ''h'' = 6.626 × 10-34 J·s is Planck's Constant , and ''ν'' is the frequency of the wave. One rule is always obeyed regardless of the circumstances: EM radiation in a vacuum always travels at the Speed Of Light , ''relative to the observer'', regardless of the observer's velocity. (This observation led to Albert Einstein 's development of the theory of Special Relativity .) In a medium (other than vacuum), Velocity Of Propagation or Refractive Index are considered, depending on frequency and application. Both of these are ratios of the speed in a medium to speed in a vacuum. ELECTROMAGNETIC SPECTRUM See Also: electromagnetic spectrum |
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