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.

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:

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.