Intensity is power transferred per unit area perpendicular to energy propagation. SI units are watts per square metre (W/m²) or kg⋅s−3. Not synonymous with strength, amplitude, or level
Longitudinal waves vibrate parallel to wave direction and produce compression. Examples include sound waves and seismic P waves. Some authors use "L-waves" and "T-waves" for convenience
Signals travel as electromagnetic waves at 50-99% of light speed in vacuum. Cables act as waveguides, guiding energy-carrying waves. Electromagnetic waves enter conductors within milliradian of surface. Copper at 60 Hz has velocity of 3.2 m/s
Hertz proved electric current has negligible mass at Berlin University in 1878. He earned doctorate in physics at age 23 through electromagnetic induction research. He reworked Maxwell's equations and concluded Maxwell's theory was most promising
Radiation is the flow of particles and waves at speeds above light speed. Matter is constantly bombarded with radiation from cosmic and terrestrial sources. Radiation can exhibit both wave-like and particle-like behavior
Wave is a disturbance that travels through material medium without matter transfer. Particles vibrate about mean positions without permanent displacement. Waves transfer energy and propagate without clear boundaries