# Frequency

number of occurrences or cycles per time

Frequency is how often an event repeats itself over a set amount of time. As time elapses—here moving left to right on the horizontal axis—the five sinusoidal waves vary, or cycle, regularly at different rates (or ratios). The red wave (top) has the lowest frequency (i.e. cycles at the slowest rate) while the purple wave (bottom) has the highest frequency (cycles at the fastest rate). Sines with three different frequencies f.

In physics, the frequency of a wave is the number of wave crests that pass a point in one second (a wave crest is the peak of the wave).

Hertz (symbol Hz) is the unit of frequency.

The relationship between frequency and wavelength is expressed by the formula:

$f=v/\lambda$ where v is speed and $\lambda$ (lambda) is wavelength. The formula for the frequency of light waves is $f=c/\lambda$ where the c is the speed of light.

All electromagnetic waves travel at the speed of light in a vacuum but they travel at slower speeds when they travel through a medium that is not a vacuum. Other waves, such as sound waves, travel at much much lower speeds and can not travel through a vacuum.

Examples of electromagnetic waves are: light waves, radio waves, infrared radiation, microwaves, and gamma waves.

## Visualizing electromagnetic waves

Different types of electromagnetic waves have different frequencies.