Electromagnetic WavesIntroduction
On the evening of 23 August 2023, Riya sat glued to the livestream as Chandrayaan-3's lander touched down near the Moon's south pole. Every message from the lander crossed about 3.84 lakh kilometres of empty space to reach the ground station near Bengaluru, and each one arrived just 1.3 seconds after it was sent.
What could cross all that emptiness, with no air, no water and no wire in between, so fast? Before we answer, look at what the space between the Moon and the earth lets through. Sort these by whether they can travel across empty space.
Sound and ripples are vibrations of a material, so they stop where the material ends. Everything in the first group is the same kind of wave in disguise: an electromagnetic wave, made of electric and magnetic fields that change with time and travel on their own. In empty space all of them, from radio to X-rays, move at the same speed, about 3 × 10⁸ m/s, the speed of light. That is why the lander's 3.84 lakh km took only 1.3 s. This chapter is about where these waves come from and why they exist at all.
Maxwell's Question
In Chapter 4 you saw that an electric current makes a magnetic field, and that two wires carrying currents push or pull on each other. In Chapter 6 you saw the reverse link: a magnetic field that changes with time induces an electric field. In the 1860s James Clerk Maxwell asked whether the pattern was complete.
Which statement would complete the symmetry with Faraday's discovery?
Maxwell argued that nature does work this way: not only an electric current but also an electric field that changes with time produces a magnetic field. He was led to it by a flaw he found in Ampère's circuital law, which gives two different answers for the magnetic field near a capacitor that is being charged. To remove the flaw he added a new term, which he called the displacement current.
Maxwell then wrote the laws of electricity and magnetism as a set of four equations, now called Maxwell's equations. Together with the Lorentz force of Chapter 4 they contain all of classical electromagnetism. Their most important prediction was a surprise: electromagnetic waves, coupled electric and magnetic fields that travel through space. The speed the equations gave was very close to the speed of light measured by optics experiments, about 3 × 10⁸ m/s, and Maxwell concluded that light itself is an electromagnetic wave. Electricity, magnetism and light became one subject. Heinrich Hertz made and detected such waves in his laboratory in 1887, and Marconi and others soon turned them into the communication revolution we live in today.
Note
James Clerk Maxwell (1831–1879), born in Edinburgh, Scotland, was one of the greatest physicists of the nineteenth century. He worked out how the speeds of molecules in a gas are spread out, and was among the first to estimate the size of molecules from quantities such as viscosity. His greatest achievement was to unite the laws of Coulomb, Oersted, Ampère and Faraday into one consistent set of equations. Curiously, he did not accept the idea, strongly suggested by Faraday's laws of electrolysis, that electricity comes in particles.
In this chapter you will
- see why Ampère's law needed Maxwell's displacement current, and what it does,
- find out what makes electromagnetic waves and what they look like: E, B and the direction of travel all at right angles,
- and tour the electromagnetic spectrum, from gamma rays about 10⁻¹² m long to radio waves 10⁶ m long.