Frequency is the rate at which an entity, such as a star, completes cycles of rotation. The frequency of a wave refers to how many times it oscillates in a given time. The number of cycles, or oscillations, per second is usually given the unit Hertz or Hz. Thus light, being wave-like in nature, can have its frequency measured. Many atoms and molecules emit light with well-defined frequencies.

Sine waves
Sine waves of different frequency. The orange curve has the lowest frequency, while the red curve has the highest frequency.

Stars like pulsars rotate very rapidly, and astronomers often talk about their rotation frequencies.

The inverse of frequency, f is the period, P:

f = 1/P

The velocity, v of a wave is the frequency multiplied by its wavelength, λ, ie

v = f λ

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