Electromagnetic Waves | Class 12 Physics Notes

8.1 INTRODUCTION

In earlier chapters, we learned that an electric current produces a magnetic field and that two current-carrying wires exert forces on each other due to their magnetic fields. Additionally, a changing magnetic field produces an electric field, as seen in Faraday's law of electromagnetic induction. The question arises: does a changing electric field also produce a magnetic field? James Clerk Maxwell answered this affirmatively, proposing that not only electric currents but also time-varying electric fields generate magnetic fields. While applying Ampere's circuital law to a charging capacitor, Maxwell noticed an inconsistency. The law, as originally formulated, did not account for the magnetic field in the region between the capacitor plates where no conduction current flows. To resolve this, Maxwell introduced the concept of displacement current, an additional current term arising from the changing electric field. This addition made Ampere's law consistent and symmetric with Faraday's law.

Maxwell formulated a set of four equations, now known as Maxwell's equations, that unify electricity, magnetism, and optics. These equations describe how electric and magnetic fields are generated by charges, currents, and changes in each other. One of the most profound predictions from Maxwell's equations is the existence of electromagnetic waves—coupled oscillations of electric and magnetic fields that propagate through space at a speed very close to the speed of light (3 × 10^8 m/s). This led to the remarkable conclusion that light itself is an electromagnetic wave, unifying the previously separate domains of electricity, magnetism, and optics. Heinrich Hertz experimentally demonstrated electromagnetic waves in 1885, confirming Maxwell's prediction. Subsequently, the technological exploitation of these waves revolutionized communication.

This chapter begins by discussing the need for displacement current and its implications, followed by a qualitative description of electromagnetic waves and their properties. Finally, the chapter covers the electromagnetic spectrum, spanning from gamma rays with extremely short wavelengths to long radio waves with very large wavelengths.

📊 Diagram: See figure_1: James Clerk Maxwell (1831 - 1879) Born in Edinburgh, Scotland, was among the greatest physicists of the nineteenth century. He derived the thermal velocity distribution of molecules in a gas and was among the first to obtain reliable estimates of molecular parameters from measurable quantities like viscosity, etc. Maxwell's greatest achievement was the unification of the laws of electricity and magnetism (discovered by Coulomb, Oersted, Ampere and Faraday) into a consistent set of equations now called Maxwell's equations. From these he arrived at the most important conclusion that light is an electromagnetic wave. Interestingly, Maxwell did not agree with the idea (strongly suggested by the Faraday's laws of electrolysis) that electricity was particulate in nature.

🔗 Connection: This introduction sets the stage for the next section on displacement current, explaining why Maxwell introduced this concept to resolve inconsistencies in Ampere's law.