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MeasurementTo calculate the frequency of the event, the number of occurrences of the event within a fixed time interval are counted, and then divided by the length of the time interval.
In SI units, the result is measured in hertz (Hz), named after the German physicist Heinrich Rudolf Hertz. 1 Hz means that an event repeats once per second, 2 Hz is twice per second, and so on. This unit was originally called a cycle per second (cps), which is still sometimes used. Other units that are used to measure frequency include revolutions per minute (rpm). Heart rate and musical tempo are measured in beats per minute (BPM). Often, angular frequency is used instead of frequency, measured in radians per second (rad/s). An alternative method to calculate frequency is to measure the time between two consecutive occurrences of the event (the period) and then compute the frequency f as the reciprocal of this time:
f = \frac{1}{T} </math> where
Frequency of wavesFrequency has an inverse relationship to the concept of wavelength. The frequency f is equal to the speed v of the wave divided by the wavelength λ (lambda) of the wave:
f = \frac{v}{\lambda} </math> In the special case of electromagnetic waves moving through a vacuum, then v = c, where c is the speed of light in a vacuum, and this expression becomes:
f = \frac{c}{\lambda} </math> When waves travel from one medium to another, their frequency remains exactly the same — only their wavelength and speed change. Apart from being modified by the Doppler effect or any other nonlinear process, frequency is an invariant quantity in the universe. That is, it cannot be changed by any linearly physical process unlike velocity of propagation or wavelength. Frequency is also a measure of temperature, as for when the temperature of a molecule is increased it will vibrate faster. The faster the molecule will vibrate the higher the frequency. Examples
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