Trough

The crest is the 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.

About toughs
  • On a wave at sea, the trough is a point where the displacement of water is at a minimum. A crest is the opposite of a trough, so a crest is a point where the displacement of the water is at a maximum.
  • In the case of an electromagnetic wave which has an electric and a magnetic axis,  a trough on either axis refers to minimum displacement in the negative direction whilst a crest refers to maximum displacement.
  • 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.
  • Wavelength refers to a complete wave-cycle from crest to crest.
  • Frequency refers to the number of wave-cycles that pass a given point in a given amount of time.
  • The greater the distance from the top of a crest or to the bottom of a corresponding trough the more energy the wave carries.
  • Radio waves are long waves and can measure thousands of yards long from crest to crest.
  • Microwaves have wavelengths of only a few millimetres.
  • The amplitude of the electric field of an electromagnetic wave is a measurement of voltage – volts per metre.

https://en.wikipedia.org/wiki/Crest_and_trough