8.3.1. Sources of electromagnetic waves

How are electromagnetic waves produced? Neither stationary charges nor charges in uniform motion (steady currents) can be sources of electromagnetic waves. The former produces only electrostatic fields, while the latter produces magnetic fields that, however, do not vary with time.

Electromagnetic waves require time-varying fields to be generated. Stationary charges, for instance, create static electric fields and stationary currents create consistent magnetic fields. Since neither causes changes in the electric or magnetic fields, they do not result in the propagation of electromagnetic waves.

Think of a calm lake where the surface is still (stationary charges) — it doesn’t create any waves. Now imagine someone throws a stone into that lake (accelerating charges) — the ripples that form and travel outwards represent the generated electromagnetic waves caused by moving charges.

It is an important result of Maxwell’s theory that accelerated charges radiate electromagnetic waves. The proof of this basic result is beyond the scope of this book, but we can accept it on the basis of rough, qualitative reasoning.

Maxwell's theory asserts that when a charge accelerates, it disturbs the surrounding electric field, creating oscillating electric and magnetic fields. These oscillations regenerate each other, allowing the wave to propagate through space. The frequency of the resulting electromagnetic wave corresponds to the frequency of the charge's oscillations.

Consider a child on a swing (the charge) moving back and forth. As the swing accelerates at the limits of its path, it throws up ripples in the air (the electromagnetic waves), with the frequency of those ripples mirroring the speed and frequency of the swing’s motion.

The energy associated with the propagating wave comes at the expense of the energy of the source – the accelerated charge.

When an accelerated charge generates electromagnetic waves, it loses energy. This energy is carried away by the waves as they travel through space. The amount of energy emitted is proportional to the acceleration of the charge; greater acceleration results in stronger waves.

Imagine a person running and waving their arms enthusiastically. As they expend energy to wave their arms, they make the air around them move in waves, much like how a charge emits energy as an electromagnetic wave. If they tire, they might slow down their movements, producing weaker air waves.

From the preceding discussion, it might appear easy to test the prediction that light is an electromagnetic wave. We might think that all we needed to do was to set up an ac circuit in which the current oscillate at the frequency of visible light, say, yellow light. But, alas, that is not possible. The frequency of yellow light is about 6 × 1014 Hz, while the frequency that we get even with modern electronic circuits is hardly about 1011 Hz.

Testing the generation of electromagnetic waves at the frequency of visible light is impractical with current technology. Current alternating current (AC) circuits operate at much lower frequencies, making it difficult to replicate the high frequency necessary for visible light. This challenge illustrates the limits of human technology in matching natural phenomena.

It’s akin to trying to play a high-pitched note on a piano that doesn’t have the higher keys. Just as the piano can only produce certain tones (lower frequencies), our electronic circuits can’t reach the higher frequencies of light.

Hertz’s successful experimental test of Maxwell’s theory created a sensation and sparked off other important works in this field. Two important achievements in this connection deserve mention. Seven years after Hertz, Jagdish Chandra Bose, working at Calcutta (now Kolkata), succeeded in producing and observing electromagnetic waves of much shorter wavelength (25 mm to 5 mm). His experiment, like that of Hertz’s, was confined to the laboratory. At around the same time, Guglielmo Marconi in Italy followed Hertz’s work and succeeded in transmitting electromagnetic waves over distances of many kilometres.

Hertz's experiments proved the existence of electromagnetic waves, aligning with Maxwell's theories. Following this, Bose produced shorter wavelength waves in a laboratory setting, while Marconi's advancements in radio technology demonstrated practical applications, showing the waves could travel significant distances.

Hertz’s work was like the first person to discover the ocean — an exciting revelation. Bose then explored the depths of different types of waves within that ocean, while Marconi built the ships (radio technology) to travel across it, allowing communication over vast distances.

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