Most waves can be defined by just a few parameters: period, frequency, wavelength, amplitude, particle velocity, phase velocity, and group velocity. ‘Waves in essence’ explains these parameters in turn and then goes on to discuss the spreading and fading of waves and the complexities of waves that arise through their interactions with objects and other waves resulting in diffraction and interference. It also describes the difference between longitudinal and transverse waves and the important wave phenomena of refraction and reflection. It then outlines the fundamental difference between pressure waves like sound, ocean, and seismic waves, and electromagnetic waves, which include light and radio waves. All electromagnetic radiation is made of particles called photons.