Wavelength and frequency

For a given wave, the relationship between frequency and wavelength is linked by its velocity. In the case of an electromagnetic wave in vacuum, its propagation velocity is equal to c, the speed of light, which is defined as 299 792 458 m/s in SI. The frequency corresponds to the number of times a wave/particle vibrates during a certain time period, or how many times it comes back to the same state per time unit. It's usually measured in Hertz (1/s). The wavelength is the distance in space between those vibrations, or the distance that separates the two closest points where the wave is in the same state. It is usually measured in meters.