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](https://kids.britannica.com/students/article/wave/628468)
**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](https://www.omnicalculator.com/conversion/time-unit-converter) 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](https://www.omnicalculator.com/conversion/cps-converter)). 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](https://www.omnicalculator.com/conversion/length-converter) of one wave cycle. **The longer the wavelength, the lower the frequency**:
Source: [Encyclopedia Britannica](https://kids.britannica.com/students/article/wave/628468))
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`.