Lecture6 (Chap 6)
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Transcript of Lecture6 (Chap 6)
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7/28/2019 Lecture6 (Chap 6)
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Electromagnetic Waves
The Nature of Electromagnetic Waves
The Energy carried by Electromagnetic Waves
The Doppler Effect and Electromagnetic Waves
Polarization
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24.1The Nature of Electrom agnetic Waves
Two straight wires connected tothe terminals of an AC generator
can create an electromagnetic
wave.
Only the electric wave traveling to the
right is shown here.
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24.1The Nature of Electrom agnetic Waves
The current used to generate
the electric wave creates a
magnetic field.
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24.1The Nature of Electrom agnetic Waves
This picture shows the wave of the radiation field far from the
antenna.
The speed of an electromagnetic wave in a vacuum is:
sm1000.3 8c
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24.1The Nature of Electrom agnetic Waves
A radio wave can be detected with a receiving antenna wire
that is parallel to the electric field.
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24.1The Nature of Electrom agnetic Waves
With a receiving antenna in the form of a loop, the magnetic
field of a radio wave can be detected.
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24.2The Electrom agnet ic Spectrum
Like all waves, electromagnetic waves have a wavelength andfrequency, related by:
fc
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24.2The Electrom agnet ic Spectrum
Examp le 1The Wavelength of Visible Light
Find the range in wavelengths for visible light in the frequency rangebetween 4.0x1014Hz and 7.9x1014Hz.
nm750m105.7Hz104.0
sm1000.3 7
14
8
f
c
nm380m108.3Hz107.9
sm1000.3 714
8
f
c
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24.3The Speed of Ligh t
The speed of l ig ht
in a vacuum
sm458792299c
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24.4The Energy Carr ied by Electrom agnet ic Waves
Electromagnetic waves, like water waves, carry energy.
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24.4The Energy Carr ied by Electrom agnet ic Waves
The total energy d ensity carr ied by an electromagn et ic
wave
22
2
1
2
1
Volume
energyTotalBEu
o
o
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24.4The Energy Carr ied by Electrom agnet ic Waves
cutA
uctA
AtA
PS
energyTotal
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24.5The Doppler Effect and Electromagnet ic Waves
Electromagnetic waves also can exhibit a Dopper effect, but it
differs for two reasons:
a) Sound waves require a medium, whereas electromagnetic
waves do not.
b) For sound, it is the motion relative to the medium that is important.
For electromagnetic waves, only the relative motion of the sourceand observer is important.
cvc
v
ff so
rel
rel
if1
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Exercise
Police use radar guns and the Doppler effect to catchspeeders. A radar gun emits an electromagnetic wavethat reflects from the oncoming car as shown in thefigure below. The reflected wave returns to the police carwith a frequency (measured by on-board equipment) that
is different from the emitted frequency. Suppose that apolice car is moving to the right at 25 m/s, while thespeeder is coming up from behind at a speed of 37 m/s,both speeds being with respect to the ground. Assumethat the electromagnetic wave emitted by the radar gun
has a frequency of 8.0 x 109 Hz. Find the differencebetween the frequency of the wave that returns to thepolice car after reflecting from the speeders car and theoriginal frequency emitted by the police car.
24 6 P l i t i
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24.6Polarizat ion
Linearly polarized wave
on a rope.
POLARIZED ELECTROMAGNETIC WAVES
24 6 P l i t i
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24.6Polarizat ion
In polarized light, the electric field
fluctuates along a single direction.
24 6 P l i t i
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24.6Polarizat ion
Polarized light may be produced from unpolarized light with
the aid of polarizing material.
24 6 P l i t i
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24.6Polarizat ion
MALUS LAW
2cosoSS
intensity before
analyzerintensity after
analyzer
24 6 Polarizat ion
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24.6Polarizat ion
Example 7Using Polarizers and Analyzers
What value of should be used so the average intensity of the polarized
light reaching the photocell is one-tenth the average intensity of the
unpolarized light?
24 6 Polarizat ion
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24.6Polarizat ion
2
2
1
10
1
cosoo SS
2
5
1 cos
51
cos
4.63
24 6 Polarizat ion
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24.6Polarizat ion
When Polaroid sunglasses are
crossed, the intensity of the
transmitted light is reduced to
zero.
24 6 Polarizat ion
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24.6Polarizat ion
IMAX movie projector
24 6 Polarizat ion
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24.6Polarizat ion
THE OCCURANCE OF POLARIZED LIGHT IN NATURE