chapter 32 electromagnetic waves - …people.physics.tamu.edu/adair/phys208/chapt32/chapter...
TRANSCRIPT
1
CHAPTER 32
ELECTROMAGNETIC WAVES
BASIC CONCEPTS
PROPAGATION OF LIGHT
ELECTROMAGNETIC SPECTRUM
ENERGY IN ELECTROMAGNETIC WAVES ‐
THE POYNTING VECTOR
3
Maxwell used these equations to predict
the propagation of electromagnetic waves,
light.
Assume an E‐M wave moving in the
direction.
4
First apply Gauss’s law
Flux is zero. This would not be
true if either had components in the
direction.
12
We have from Faraday’s Law
We have from Ampere’s Law
Thus we get (putting the second in the first)
And
13
The E‐M Wave will travel in the direction
at a speed of (where ).
and no
components in the direction.
14
ELECTROMAGNETIC WAVES
1. The solutions of Maxwell’s third and
fourth equations are wavelike where both E
and B satisfy the same wave equation.
2. Electromagnetic waves travel through
empty space with speed of light, .
3. The electric and magnetic field
components of plane electromagnetic
waves are perpendicular to each other and
perpendicular to the direction of
propagation.
4. The relative magnitudes of E and B in
empty space are related by .
5. Electromagnetic waves obey the principle
of superposition.
15
The equations for can be written.
They are
And
Defining the wave
The wavelength is the distance along
the wave from a point to the next point
where the waves starts to repeat.
The frequency is the number of times
per second a point on the wave passes
through a cycle.
17
Energy in E‐M Waves
Remember from Chapter 24 Energy of
Electric Field
And
From Chapter 30 Energy of Magnetic Field
Thus for a region with both fields
19
Consider region of space where E‐M wave
propagating in .
Energy in region defined by by will be
energy that passed through area
20
Energy in region is energy density multiplied
by volume.
The energy per unit time through will be
And the energy per unit time per unit area
will be
22
The energy per unit area per unit time
passing through an area with an E‐M wave.
We define a vector, the Poynting vector, to
be
23
The direction of the Poynting vector is the
direction of propagation of the wave.
The magnitude of the Poynting vector is the
energy per unit time per unit area (power
per unit area) delivered by the wave.
Calculate the average power delivered
(Books way).
28
TWO NEW SCIENCES
By
GALILEO
Simplicio: Everyday experience shows that the
propagation of light is instantaneous; for when
we see a piece of artillery fired, at a great
distance, the flash reaches our eyes without
lapse of time; but the sound reaches the ear
only after a noticeable interval.
Sagredo: Well, Simplicio, the only thing I am
able to infer from this familiar bit of experience
is that sound, in reaching our ear, travels more
slowly than light; it does not inform me
whether the coming of the light is
instantaneous or whether, although extremely
rapid, it still occupies time.