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Radiology => Electromagnetic Radiation
Electromagnetic Radiation
INTRODUCTION
Electromagnetic Radiation, energy waves produced by the oscillation or acceleration of an electric charge. Electromagnetic waves have both electric and magnetic components. Electromagnetic radiation can be arranged in a spectrum that extends from waves of extremely high frequency and short wavelength to extremely low frequency and long wavelength (see Wave Motion). Visible light is only a small part of the electromagnetic spectrum. In order of decreasing frequency, the electromagnetic spectrum consists of gamma rays, hard and soft X rays, ultraviolet radiation, visible light, infrared radiation, microwaves, and radio waves.
PROPERTIES
There are three phenomena through which energy can be transmitted: electromagnetic radiation, conduction, and convection (see Heat Transfer). Unlike conduction and convection, electromagnetic waves need no material medium for transmission. Thus, light and radio waves can travel through interplanetary and interstellar space from the sun and stars to the earth. Regardless of the frequency, wavelength, or method of propagation, electromagnetic waves travel at a speed of 3 × 1010 cm (186,272 mi) per second in a vacuum. All the components of the electromagnetic spectrum, regardless of frequency, also have in common the typical properties of wave motion, including diffraction and interference. The wavelengths range from millionths of a centimeter to many kilometers. The wavelength and frequency of electromagnetic waves are important in determining heating effect, visibility, penetration, and other characteristics of the electromagnetic radiation.
THEORY
British physicist James Clerk Maxwell laid out the theory of electromagnetic waves in a series of papers published in the 1860s. He analyzed mathematically the theory of electromagnetic fields and predicted that visible light was an electromagnetic phenomenon.
Physicists had known since the early 19th century that light is propagated as a transverse wave (a wave in which the vibrations move in a direction perpendicular to the direction of the advancing wave front). They assumed, however, that the wave required some material medium for its transmission, so they postulated an extremely diffuse substance, called ether, as the unobservable medium. Maxwell's theory made such an assumption unnecessary, but the ether concept was not abandoned immediately, because it fit in with the Newtonian concept of an absolute space-time frame for the universe. A famous experiment conducted by the American physicist Albert Abraham Michelson and the American chemist Edward Williams Morley in the late 19th century served to dispel the ether concept and was important in the development of the theory of relativity. This work led to the realization that the speed of electromagnetic radiation in a vacuum is an invariant.
IVQUANTA OF RADIATION At the beginning of the 20th century, however, physicists found that the wave theory did not account for all the properties of radiation. In 1900 the German physicist Max Planck demonstrated that the emission and absorption of radiation occur in finite units of energy, known as quanta. In 1904, German-born American physicist Albert Einstein was able to explain some puzzling experimental results on the external photoelectric effect by postulating that electromagnetic radiation can behave like a particle (see Quantum Theory).
Other phenomena, which occur in the interaction between radiation and matter, can also be explained only by the quantum theory. Thus, modern physicists were forced to recognize that electromagnetic radiation can sometimes behave like a particle, and sometimes behave like a wave. The parallel concept-that matter also exhibits the same duality of having particlelike and wavelike characteristics-was developed in 1925 by the French physicist Louis Victor, Prince de Broglie.
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