In SI units, the unit of frequency is Hertz (Hz), named after the German physicist Heinrich Hertz: 1 Hz means that an
event repeats once per second. A previous name for this unit was cycles per second. For engines and rotating mechanical
devices it is often called revolutions per minute, abbreviated RPM. 60 RPM equals one hertz. The period, usually denoted
by T, is the length of time taken by one cycle, and is the reciprocal of the frequency f.
The SI unit for period is the second.
For periodic waves, frequency has an inverse relationship to the concept of wavelength; simply, frequency is inversely
proportional to wavelength λ (lambda). The frequency f is equal to the phase velocity v of the wave divided by the
wavelength λ of the wave.