Every photon is characterized by wavelength (the distance from the crest of one wave to the crest of the next wave), by frequency (the number of wave cycles that pass by in a given period ...

to the next crest or from one trough (the lowest point) to the next trough. For different types of waves — such as electromagnetic waves, sound waves or water waves — the wavelength can vary ...

(The symbol is a Greek letter, 'lambda'.) To avoid confusion, it is best to measure wavelength from the top of a crest to the top of the next crest, or from the bottom of a trough to the bottom of ...