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Radiology => Electromagnetic Radiation => Frequency
Frequency
Frequency, term used in the physical sciences to denote the number of times that any regularly recurring phenomenon occurs in one second. Frequency is important in many fields of science, such as mechanics, and the study of sound waves.
Frequencies of oscillating objects can cover a wide range of values. The tremors of earthquakes may have a frequency of less than 1, while the rapid electromagnetic oscillations of gamma rays may have frequencies of 1020 or more. In almost all forms of mechanical vibration, a relationship exists between frequency and the physical dimensions of the vibrating object. Thus, the time required by a pendulum to make one complete swing is partly determined by the length of the pendulum; the frequency or speed of vibration of a string of a musical instrument is partly determined by the length of the string. In each instance, the shorter the object, the higher the frequency of vibration.
In wave motion of all kinds, the frequency of the wave is usually given in terms of the number of wave crests that pass a given point in a second. The velocity of the wave and its frequency and wavelength are interrelated. The wavelength (the distance between successive wave crests) is inversely proportional to frequency and directly proportional to velocity. In mathematical terms, this relationship is expressed by the equation V = ë f, where V is velocity, f is frequency, and ë is wavelength. From this equation any one of the three quantities can be found if the other two are known.
Frequency is expressed in hertz (Hz); a frequency of 1 Hz means that there is 1 cycle or oscillation per second. The unit is named in honor of the German physicist Heinrich Rudolf Hertz, who first demonstrated the nature of electromagnetic wave propagation. Kilohertz (kHz), or thousands of cycles per second, megahertz (MHz), or millions of cycles per second, and gigahertz (GHz), or billions of cycles per second, are employed in describing certain high-frequency phenomena, such as radio waves. Radio waves and other types of electromagnetic radiation may be characterized either by their wavelengths, or by their frequencies. Electromagnetic waves of extremely high frequencies, such as light and X rays, are usually described in terms of their wavelength measure, which is often expressed in angstrom units (Å; hundred-millionths of a cm). An electromagnetic wave that has a wavelength of 1 Å has a frequency of about 3 billion GHz.
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