1

Chapter 24: Electromagnetic Waves Physics

Objectives:

Understand the propagation of an electromagnetic wave

Describe the relationship among light, radio waves, microwaves, and x-rays

Discuss early techniques for measuring the speed of light

Creating the Electromagnetic Wave

Suppose a signal generator is connected to two pieces of wire as shown in figure (a), and an ac

signal is applied beginning at t=0. The signal creates an electric field E.

At time T/4, figure (b), the original E field

has moved to the right by a quarter

wavelength.

At time T/2, figure (c), the original E field

has moved to the right by a half wavelength.

At time T, figure (d), the original E field has

now moved a full wavelength to the right.

The vertical wires attached to the signal

generator make an ____________________.

From our discussion on magnetism, we showed that a moving electric charge (electric current)

creates a magnetic field that is perpendicular to the direction of the current. The same holds true

for an electromagnetic wave.

2

When we studied induction, we showed that a changing magnetic field produces an electric

potential.

As a result of these two phenomena, the electromagnetic wave can propagate great distances by

alternately creating electric and magnetic fields.

Much of the credit behind the theory of electromagnetic waves goes to Michael Faraday

(England, 1791-1867) and James Clerk Maxwell (Scotland, 1831-1879).

The Electromagnetic Spectrum

The collection of electromagnetic waves from the longest wavelength to the shortest (lowest

frequency to the highest) is referred to as the electromagnetic spectrum.

The Speed of Light

Electromagnetic waves are known to travel at the speed of light (3 × 108 m/s). For many years,

scientists attempted to develop techniques for measuring its speed. An American, Albert

Michelson (1852-1931) determined the first truly accurate estimate of its speed. As a result, he

was the first American to receive the Nobel Prize in Physics (1907).