Lecture 11 Electromagnetic Waves

Chapter 21.8  21.13

Outline

• Discovery and Studies of Electromagnetic Waves • Properties of Electromagnetic Waves • The Spectrum of Electromagnetic Waves

Discovery of EM Waves

A changing magnetic field generates an electric current  electromagnetic induction.

J.C.Maxwell proposed that a changing electric field has an associated magnetic field.

Such a combined effect results in existence of electromagnetic waves , which can travel indefinitely in empty space (vacuum).

Electromagnetic waves travel with the speed of light.

Properties of EM Waves

The electric and magnetic fields of a wave are perpendicular to each other and to the travel direction.

EM waves are transverse waves.

EM waves travel with the speed of light (

c)

.

EM waves carry both energy and momentum .

For an EM wave: 1 c =   (  0 є 0 ) = 2.998 10 8 m/s c = E/B Average power per unit area E max B max E max 2 c B max 2  2  0 =  2  0 =  c 2  0

Problem with EM waves

Problem:

An EM wave in vacuum has an electric field amplitude of 300 V/m.

Find

the amplitude of the corresponding magnetic field.

c =

E

/

B



B

=

E

/c

B

= 300 V/m / 3 10 8 m/s = 10  6 T = 1  T

Properties of EM Waves

All EM waves travel through space with a speed

c

 their wavelength  and frequency

f

are related: Demonstration 

f

= c The wavelength is the distance between adjacent peaks of the electric or magnetic field 1 nm (nanometer) = 10 –9 m 1μm (micron) = 10 –6 The frequency is the number of peaks that pass by any point each second, measured in cycles per second or Hertz (Hz).

light demo m

The Spectrum of EM Waves

The total variety of EM waves is called the electromagnetic spectrum.

Different portions of the spectrum are called: Gamma rays X rays  the shortest wavelength light - wavelengths shorter than ultraviolet The ultraviolet light - beyond the blue end of rainbow The visible light what we see with our eyes The infrared light - beyond of the red end of rainbow Radio waves light with the longest wavelengths

Electromagnetic spectrum

E = h 

Electromagnetic spectrum

E = energy h = Planck constant  = frequency

Summary

• Electromagnetic waves are fluctuations generated by moving charges that produce fluctuating electric and magnetic fields.

• Visible light is only a small portion of the electromagnetic spectrum.