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Waves of different frequencies ( in light these are different colours ) can interfere to form a pulse if they have a fixed relative phase-relationship ( see Fourier transform ).
Conversely, if waves of different frequencies are not coherent, then, when combined, they create a wave that is continuous in time ( e. g. white light or white noise ).
The temporal duration of the pulse is limited by the spectral bandwidth of the light according to:

1.826 seconds.