Wavelength
It is the distance between successive crests of a wave, especially points in a soundwave or electromagnetic wave. Frequency is the rate at which the it is repeated over a particular period of time. Trough is the level at which the wave has the highest point. Crest on the other hand is the opposite of trough it is the lowest point of wave. The unit of frequency is the Hertz. It is the SI unit of frequency, equal to one cycle per second.
