The wavelength is the distance of neighbouring oscillation states of the same phase in the direction of propagation, e.g. between two wave crests following upon each other. Wavelength and frequency are connected to each other.

The measure of wavelength is metre (m):

1 millimeter (mm) = 1/1000 m
1 mikrometer (µm) = 1/1000 mm
1 nanometer (nm) = 1/1000 µm