Nature of Electromagnetic Waves

• Electromagnetic radiation is a disturbance in an electric field.

• This results in a magnetic field at right angles to the electric one.

• Therefore it is called an electromagnetic radiation – electric and magnetic disturbances result!

It propagates (moves) through space carrying energy from one place to

another

• All electromagnetic waves (also called electromagnetic photons) can travel through a vacuum – it is the only way that energy can propagate through space

• Wavelength is the smallest distance between two corresponding points on a wave.

• It is given the symbol and is measured in metres.

The wave equation

wave speed (metre/second, m/s) = frequency (hertz, Hz) × wavelength (metre, m)

wavespeed = f

• All electromagnetic radiation travels through a vacuum at the same speed – the speed of light – symbol ‘c’ – which is 300,000,000 m/s

• This is the fastest speed anything can travel at!

• As electromagnetic waves all travel at the same speed and:

c = f• If a wave has a long wavelength it

must have a low frequency and vice versa a high frequency wave must have a short wavelength… f and have to always equal 300,000,000 when they are multiplied together.

Electromagnetic waves

2

2

22

t

EB

t

EEEE

o

02

22

t

EE o

or,or,

For the electric field E,For the electric field E,

i.e. wave equation with vi.e. wave equation with v22 = 1/µ = 1/µoo

Electromagnetic waves0

2

22

t

BB o

Similarly for the magnetic fieldSimilarly for the magnetic field

i.e. wave equation with vi.e. wave equation with v22 = 1/µ = 1/µoo

oo

c1

c = 3.0 X 10c = 3.0 X 1088 m/s m/s

Solution of the equation

Irradiance (energy per unit volume)

• Energy density stored in an electric field

• Energy density stored in a magnetic field

32 ,

2

1

m

JEu oE

32 ,

2

1

m

JBu

oB

c

EB

32

2

2 2

1

2

1

m

JE

c

Eu o

oB