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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
=  f, where V is velocity,
f is frequency, and 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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