Definitions and Units

  • Frequency is the number of occurrences of a repeating event per unit of time.
  • Temporal frequency is used to distinguish frequency from spatial frequency.
  • Ordinary frequency is related to angular frequency by a factor of 2π.
  • The period is the interval of time between events, and it is the reciprocal of the frequency.
  • The SI unit of frequency is the hertz (Hz), and the SI unit for the period is the second.

Period versus Frequency

  • Frequency is the number of cycles or repetitions per unit of time.
  • The period is the time taken to complete one cycle of an oscillation or rotation.
  • Temporal frequency emphasizes the number of occurrences of a repeating event per unit time.
  • Rotational frequency is the instantaneous rate of change of the number of rotations with respect to time.
  • Angular frequency is the rate of change of angular displacement or the rate of change of the phase of a sinusoidal waveform.

Wave Propagation

  • Frequency has an inverse relationship to the wavelength in nondispersive media.
  • The frequency of a sinusoidal wave is equal to the phase velocity divided by the wavelength.
  • In vacuum, the frequency of electromagnetic waves is equal to the speed of light divided by the wavelength.
  • Monochromatic waves maintain their frequency when traveling between different media.
  • Wave period is more commonly used to describe longer and slower waves, such as ocean surface waves.


  • Frequency can be measured by counting the number of occurrences within a specific time period and dividing by the period.
  • Gating error introduces a random error into the count and causes an average error in the calculated frequency.
  • A resonant-reed frequency meter was used in the past to measure the frequency of alternating current.
  • Stroboscopes can be used to measure the frequency of rotating or vibrating objects by adjusting the flashing light frequency.
  • Higher frequencies are usually measured with a frequency counter, which uses digital logic to count the number of cycles during a time interval.
  • Frequency can be measured using physical phenomena such as the oscillation of a pendulum.
  • Sound waves can be analyzed using Fourier analysis to determine their frequency components.
  • Frequency can be estimated by observing the Doppler shift in waves emitted by a moving source.
  • Frequency can be measured using interferometry techniques.
  • Frequency can be determined by analyzing the spectrum of a signal using a spectrum analyzer.


  • Visible light is an electromagnetic wave with a frequency range of 400 THz to 800 THz.
  • Infrared (IR) radiation has frequencies below 400 THz and is invisible to the human eye.
  • Ultraviolet (UV) radiation has frequencies above 800 THz and is also invisible to the human eye.
  • Microwaves and radio waves have lower frequencies than visible light.
  • X-rays and gamma rays have higher frequencies than visible light.
  • Sound propagates as mechanical vibration waves of pressure and displacement.
  • The audible frequency range for humans is typically between 20 Hz and 20,000 Hz.
  • The frequencies of sound determine its pitch and timbre.
  • Different species have different hearing ranges, such as dogs perceiving vibrations up to 60,000 Hz.
  • In Europe, Africa, Australia, southern South America, most of Asia, and Russia, the frequency of household electrical outlets is 50 Hz.
  • In North America and northern South America, the frequency of household electrical outlets is 60 Hz.
  • The frequency of the hum in an audio recording can indicate the region where it was made.
  • Utility frequency refers to the frequency of alternating current in household electrical outlets.
  • The European frequency is close to the tone G, while the North American frequency is between the tones B and B.
  • Aperiodic frequency refers to the rate of incidence or occurrence of non-cyclic phenomena.
  • It is expressed as a reciprocal second (s) or becquerels in the case of radioactivity.
  • Aperiodic frequency is a physical quantity that involves counting the number of entities or events during a given time duration.
  • It is the inverse of the (temporal) period.
  • Aperiodic frequency is applicable to random processes like radioactive decay.

Frequency Data Sources

Reference URL
Knowledge Graph