A wave is a vibration that transfers energy but not matter.    The frequency of a wave is the number of waves that pass a given point in one second.       Frequency is measured with the unit hertz: Hz.                                      The amplitude of a wave is the distance a wave rises or falls from its usual resting position.     Wavelength is the distance between the same points on two adjacent disturbances.      Wavelength is measured in metres: m.   Transverse wave       A wave that has its vibrations at right angle to its direction is a transverse wave.       Electromagnetic waves and seismic S-waves are transverse waves.     Longitudinal wave      A wave that has its vibrations parallel to its direction is a longitudinal wave.      Sound, ultrasound and seismic P-waves are longitudinal waves.