LIGHT AS AN ELECTROMAGNETIC WAVE

Later, in the 1860s, Scottish physicist James Clerk Maxwell identified light as an electromagnetic wave. Maxwell had derived a wave form of the electric and magnetic equations, revealing a wave-like nature of electric and magnetic fields that vary with time.

Maxwell figured out that an electric field that varies along space generates a magnetic field that varies in time and vice versa. For that reason, as an oscillating electric field generates an oscillating magnetic field, the magnetic field in turn generates an oscillating electric field, and so on. Together, these oscillating fields form the electromagnetic wave shown in Figure 8. The way in which an electromagnetic wave travels through space is described by its wavelength λ, while its oscillation in time is described by the wave’s frequency. The frequency f and the wavelength are related through c = λ f, where c is the speed of light.

Figure 8 An oscillating electric field generates an oscillating magnetic field; the magnetic field in turn generates an oscillating electric field, and so on. Together these oscillating fields form an electromagnetic wave with wavelength λ that propagates at the speed of light c.

Because the speed of Maxwell’s electromagnetic waves predicted by the wave equation coincided with the measured speed of light, Maxwell concluded that light itself must be an electromagnetic wave. This fact was later confirmed experimentally by Heinrich Hertz in 1887. Today, we use the electromagnetic spectrum at all wavelengths—from the enormously long waves that we use to transmit AC power, through the radio wavelengths that are the foundation of our wireless society, to the extremely short wavelengths of gamma radiation (Figure 9).

Figure 9 The electromagnetic spectrum. Maxwell concluded that light itself must be an electromagnetic wave. This fact was later confirmed experimentally by Hertz in 1887.

We understand that the only difference between visible light and the rest of the spectrum is that it is the range of electromagnetic waves to which our eyes are sensitive.