A crest is a point on a wave with the maximum value of upward displacement within a wave-cycle. A trough is the opposite of a crest, so the minimum or lowest point in a wave-cycle.

  • On a wave at sea, the crest is a point where the displacement of water is at a maximum. A trough is the opposite of a crest, so a trough is a point where the displacement of the water is at a minimum.
  • In the case of an electromagnetic wave which has an electric and a magnetic axis,  a crest on either axis refers to maximum displacement in the positive direction whilst a trough refers to minimum displacement.
  • Wavelength refers to a complete wave-cycle from one crest to the next, or one trough to the next.
  • Frequency refers to the number of wave-cycles that pass a given point in a given amount of time.
  • The amplitude of a wave is a measurement of the distance from the centre line (or the still position) to the top of a crest or to the bottom of a corresponding trough.
  • Amplitude is related to the energy a wave carries. The energy a wave carries is related to frequency and amplitude. The higher the frequency, the more energy, and the higher the amplitude, the more energy.