Electromagnetic radiation
From Encyclopædia
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[edit] ELECTROMAGNETIC SPECTRUM
Electromagnetic radiation is most simply characterized by its frequency or wavelength. When electromagnetic waves are ordered in accordance with their frequency or wavelength, this ordered array is called the electromagnetic spectrum. A source of radiation such as the Sun, a flame, an electrical discharge, or an incandescent solid never produces just one frequency of electromagnetic wave, but rather emits a mixture of waves of many different frequencies. These spectra may be resolved, or separated, by instruments such as prisms or grating spectrometers.In principle the electromagnetic spectrum extends from zero, the short wavelength limit of the gamma-ray end of the spectrum, to infinity, the long wavelength limit of the radio end of the spectrum. Visible light, the portion of the spectrum to which the eye is sensitive, occupies a narrow band extending from about 3.8/10,000,000 to 7.5/10,000,000 meters. The colored components of this segment of the spectrum may be seen when a narrow beam of sunlight is passed through a glass prism to form a band of colors that extends from red at the long wavelength end to violet at the short wavelength end. The radiation dispersed by the prism actually extends farther in both directions but light outside of this range cannot be detected by the human eye.
[edit] SOURCES OF ELECTROMAGNETIC RADIATION
The sources of electromagnetic radiation are accelerated electrical charges and oscillating currents. For example, the current that flows back and forth in the antenna of a radio transmitter radiates electromagnetic waves in the radio frequency part of the spectrum; these wavelengths lie in the range from a few centimeters to hundreds of meters. Similarly, the oscillatory motion of electrons in atoms and molecules radiates light waves with wavelengths that may be in the infrared region (1/1,000 to 1/1,000,000 meters), the visible region, the ultraviolet region (3/10,000,000 to 1/10,000,000,000/meters), or the X-ray region (1/100,000,000 to 1/100,000,000,000 meters). The wavelength of the emitted radiation depends on the energy of the oscillations that give rise to it.Visible and ultraviolet radiation arises when the outermost and least strongly bound electrons change their atomic energy levels. X rays are produced when atoms are bombarded by high-energy electrons that can strip a strongly bound electron from an orbit close to the nucleus. When an outer electron jumps into the vacancy, a high-energy PHOTON (the particle equated with radiation) with a wavelength in the X-ray region of the spectrum is emitted.Gamma rays arise from transitions between energy levels of the nucleus. The nucleus is composed of protons and neutrons held together by nuclear forces that are much stronger than the electrical forces that bind the electrons in the atom. Because of the greater strength of the nuclear forces, the energy levels of the nucleus are widely separated, and the photons emitted as a consequence of nuclear transitions are more energetic than those emitted in atomic transitions. Gamma rays are also emitted in collisions between very high-energy charged particles such as cosmic rays colliding with nuclei of atoms of the atmosphere.The energy levels associated with the rotational and vibrational motion of molecules are closely spaced. Therefore, the photons emitted in transitions between these levels have small energies, and their wavelengths lie in the infrared part of the spectrum. Also, infrared radiation is emitted from solids, because the energy levels associated with lattice vibrations are so closely spaced as to virtually form a continuum.Radio waves are produced by coherent motion of electrons such as the oscillatory current that flows in the antenna of a radio transmitter. Since the early days of radio, the ingenuity of engineers has continually pushed the useful range of radio wavelengths Down until today the lower limit lies in the millimeter range (microwaves), where it overlaps with the infrared spectrum. Radio waves are also produced by charged particles orbiting in magnetic fields. This is the source of much of the radio-frequency radiation observed by radio astronomers. Another important source of astronomical radio frequency radiation is a transition between two closely spaced energy levels in the hydrogen atom. In this transition the orientations of the spins of the electron and proton change from antiparallel to parallel with the emission of a photon of 21-cm wavelength.
[edit] THE ELECTROMAGNETIC FIELD
The theory of electromagnetic fields was developed by James Clerk MAXWELL of Scotland and published in 1865. His work was the culmination of a long series of experiments and theoretical research performed by numerous illustrious scientists, including William GILBERT, Benjamin FRANKLIN, Charles Augustin de COULOMB, Andre AMPERE, Georg OHM, and Michael FARADAY. (For a history of the development of electromagnetic theory, see electricity.)Maxwell presented a set of equations that completely describe the electromagnetic field, how it is produced by?charges and currents, and how it is propagated in space and time. The electromagnetic field is described by two quantities, the electric component E and the magnetic component B, both of which change in space and time.One of the solutions to Maxwell's equations is a plane wave traveling in the direction of the x-axis. If one examines a narrow region of space (fixed x) while the wave transverses it, the electric component oscillates in strength with the period one divided by the frequency. Examining the entire wave at any given instant (fixed t) reveals that the wave oscillates sinusoidally in space with the period l.Accompanying the electric component is a magnetic component. The amplitude of the oscillating magnetic component is equal to that of the electric component. B is perpendicular to both E and the direction of propagation. In addition, B and E are in phase; that is, they both are at maximum amplitude at the same time.It may be shown that electromagnetic waves transport energy as well as carry momentum. It may also be shown that any accelerated charge, not necessarily a sinusoidally oscillating one, loses energy in the form of electromagnetic waves.
[edit] INTERACTION WITH MATTER
Electromagnetic waves are modified as they pass through a material medium. The medium may be a solid, liquid, gas, or plasma. (A plasma is an ionized gas, that is, a gas at a sufficiently high temperature so that the violent collisions of the atoms have dislodged one or more electrons from each atom.) As the wave propagates through the medium, each charged particle experiences a force that causes it to oscillate with the frequency of the wave. The oscillating charges modify the fields E and B. Consequently, the propagation characteristics of the wave are changed. One of these changes is simply the change in the velocity of propagation from the velocity in a vacuum c = 300,000,000 m/sec to the velocity v = c/n, where n is called the index OF REFRACTION and is characteristic of the medium. A consequence of this altered velocity is the phenomenon of REFRACTION. When a beam of light passes through a boundary separating two media of different indices of refraction (air and glass, for instance), the change in the velocity necessitates a change in the direction of propagation. The relation between these changes, known as Snell's law, (see REFRACTION) provides a means of experimentally measuring indices of refraction. It is the basis of much of the field of geometrical optics.Generally, the charged particles of a medium will respond differently to different frequencies, and as a consequence the index of refraction n will be a function of the frequency of the wave. If a beam of light that is a mixture of waves of different frequencies crosses a boundary between two media, the waves will be refracted through different angles. For example, if light from the Sun is reduced to a narrow beam and passed through a glass prism, the beam will be spread apart as it is resolved into its different spectral components. The visible light of longest wavelength, which the eye sees as red, is deviated the least, and the visible light of shortest wavelength, blue, is deviated the most. This phenomenon is known as DISPERSION.In nonconducting gases, liquids, and solids the electrons are tightly bound in atoms and are only slightly displaced by the field of an electromagnetic wave. For such media the index of refraction is greater than, but close to, unity, the index of refraction of a vacuum. For example, for light in the visible part of the spectrum, n = 1.00029 for air, n = 1.333 for water, and n = 1.5 to 1.9 for glass.