How is frequency measured?

Hint: Frequency is the number of cycles or oscillations completed in unit time. It is inversely proportional to the wavelength. Frequency is also the reciprocal of time period. It is also directly related to the angular velocity. Frequency is a characteristic of waves.

Complete step-by-step solution:
Frequency is the number of oscillations or the number of cycles completed in one second. Its SI unit is Hertz (Hz).
Therefore different ways to calculate the frequency, one of them is frequency is the reciprocal of time period. Therefore,
$\nu =\dfrac{1}{T}$
Here, $\nu $ is the frequency
$T$ is the time period
The frequency is inversely proportional to the wavelength, therefore,
$\nu \propto \dfrac{1}{\lambda }$
Here, $\lambda $ is the wavelength
On removing the proportionality sign, we get,
$\nu =\dfrac{c}{\lambda }$
Here, $c$ is the speed of light and its value is $3\times {{10}^{8}}m{{s}^{-1}}$
Frequency is also related to the angular velocity as-
$\nu =\dfrac{\omega }{2\pi }$
Here, $\omega $ is the angular velocity
Therefore, the frequency is related to time period, wavelength and angular velocity and hence can be calculated using any of the following rules.
Additional information: A wave is a disturbance in space. It propagates forward by transmitting the vibrations to the other molecules. There are two types of waves; transverse waves and longitudinal waves. The transverse waves do not require medium to propagate but the longitudinal waves cannot propagate without a medium.

Note: The wavelength is the distance between two similar points on a wave. Since it is a distance hence its SI unit is metres. Frequency, wavelength, time period etc are all characteristics of waves. A wave is a period occurrence which repeats itself after a time interval.