The understanding of electromagnetic theory in the 19th century led to the development of the wireless telegraph and eventually to the development of radio, television, and cell phones. A whole new world of communication evolved that has profoundly changed the world we live in. The use of electromagnetic radiation, or waves, for therapeutic purposes began in the early 20th century when physicians began using high-frequency currents to heat muscles and joints.1 The penetration of electromagnetic waves through body tissues enabled deeper heating than superficial heating devices (such as hot packs applied to the skin). Eventually, the development of devices that produce electromagnetic waves popularized what became known as diathermy. Originally diathermy included "ultrasonic" therapy (ultrasound);2 however, the therapeutic effect of ultrasound is produced by mechanical vibrations, not electromagnetic waves (see Chapter 4). Today diathermy includes only therapeutic devices that produce electromagnetic waves from the microwave and radio frequency sections of the electromagnetic spectrum.
The use of electromagnetic waves from the ultraviolet part of the electromagnetic spectrum to treat skin problems became popular in the 20th century. During the latter part of the century, the development of lasers enabled the use of visible light (which is also an electromagnetic wave) for surgery and therapeutic purposes.
This chapter will include the use of light from the visible radiation part of the electromagnetic spectrum (lasers and light-emitting diodes), radio frequency and microwave frequency waves (diathermy), and pulsed electromagnetic fields (PEMFs). The use of ultraviolet radiation for therapeutic purposes is covered in Chapter 15.
Electromagnetic waves are waves of energy that are propagated through space. Accelerating electric charges produce electromagnetic waves of moving electric and magnetic fields. The electric and magnetic field strengths in an electromagnetic wave are illustrated in Figure 6-1. Notice that the electric and magnetic fields at any point of the electromagnetic wave are perpendicular to each other and to the direction of the wave's motion. These waves travel through space at the speed of light.3
Alternating electric and magnetic fields surrounding an electromagnetic wave. Note that the electric and magnetic fields are perpendicular to each other and to the direction of the wave. (Art concept by Sara Monath.)
Electromagnetic waves are grouped according to their quantum energy, frequency, and wavelength, known as the electromagnetic spectrum (Fig. 6-2). Lower-frequency waves are known as radio waves. These frequencies are used to transmit radio and television signals. Cellular phones, radar, and microwave ovens use higher frequencies in the "microwave" frequency range. Diathermy devices produce electromagnetic waves from either the shortwave or microwave frequency ranges. Lasers produce electromagnetic waves from the visible light frequency range. Much higher frequencies are used to produce ultraviolet light and x-rays for diagnostic purposes.