Chapter 22: Electromagnetic Waves

Lecture 11

waves are usually thought of as an

excitation of some medium

a disturbance at one location propagates to

other location

a disturbance transports

energy and momentum from one place to another

waves can either be 

transverse or longitudinal depending on how the medium vibrates 

if the vibrations are perpendicular to the direction of travel, the wave is

transverse

if the vibrations are parallel to the direction of travel, the wave is

longitudinal

waves repeat in

time and space

wavelength characteristics the

distance over which the wave repeats in space, and is related to the wave number (k) 

Period (T) characterizes the

duration over which the wave repeats in time, and is related to both frequency and angular frequency

the simplest type of wave is a

sinusoidal wave, known as a plane wave

for a plane wave, the sign between the first two terms depends on

the direction the wave is traveling

plane wave: for a wave traveling in the positive x direction, its the

minus sign

plane wave: for a wave traveling in the negative x direction, its the

plus sign

Maxwell showed that all electric and magnetic phenomena could be described using 

only four equations involving electric and magnetic field 

vacuum - a magnetic field is produces by an

electric current and also by a changing electric field

Maxwell’s equations - electric fields are produced by

• electric charges

• changing magnetic fields 

maxwell’s equations - magnetic monopoles

do not exist

Maxwell’s equations - magnetic fields are produced by

• electric currents

• changing electric flux 

in vacuum, away from all charges and currents, they become:

• so if we can produces a changing magnetic field, we will get an electric field

• if that electric field is also changing, we will get another magnetic field 

• this process turns out to be coherent and leads to the existence of electromagnetic waves 

electromagnetic waves are different that other waves in that they do not require

a medium 

electromagnetic waves are 

• not excitations of some material thing 

• excitations of electric and magnetic fields 

electromagnetic waves consist of

oscillating electric and magnetic fields that propagate through space at the speed of light 

the electric and magnetic fields oscillate in space and time 

• with the same wavelength, frequency, and phase

• but with different amplitudes

Maxwell;s equations predict the

existence of electromagnetic waves

the electric and magnetic fields are transverse waves, meaning that the

electric and magnetic fields are always perpendicular to the direction the wave is traveling 

we classify and and name different wavelengths through the

electromagnetic spectrum

electromagnetic spectrum - as wavelength decreases,

frequency increases

the speed of the wave is determined by

the medium through which it propagates

in vacuum, all electromagnetic waves travel at the

same speed, known as the speed of light 

in matter, the speed of the wave depends on

the index of refraction of the medium, n

electromagnetic wave frequency does

not change (assuming no energy loss) and it is independent of the material