Frequency definition and the frequency formula

Source: https://www.omnicalculator.com/physics/frequency 

 Have a look at the following model of a wave; it will help you understand the terms used in the frequency definition below it. 

 

 

 Source:  Encyclopedia Britannica 

 

 

 Frequency is the number of completed wave cycles per second . In other words, frequency tells us how many wave crests pass a given point in a second. 

 This frequency definition leads us to the simplest  frequency formula : 

 f = 1 / T . 

 f  denotes frequency and  T  stands for the  time  it takes to complete one wave cycle measured in seconds. 

 The SI  frequency unit is Hertz (Hz) , which equals 1/s (one  cycle per second ). Other frequency units include millihertz (mHz), kilohertz (kHz), megahertz (MHz), gigahertz (GHz), and terahertz (THz). 

 Frequency equation from the wavelength 

 Have a look at another picture which will allow us to see that frequency is connected to wavelength. Wavelength is the distance between two adjacent crests (or troughs). In other words - it is the  length  of one wave cycle.  The longer the wavelength, the lower the frequency : 

 

 

 Source:  Encyclopedia Britannica ) 

 

 

 Another fact we need - how fast the waves travel at (wave velocity) determines how many of them will pass a given point per second. This means  the higher the wave velocity, the higher the frequency . 

 These two relationships between frequency and wavelength (λ), and between frequency and velocity (v), bring us to the following  frequency equation : 

 f = v / λ .