PHY138 – Waves, Lecture 6
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Today’s overview
Waves in the Electric and Magnetic Fields
The Electromagnetic Spectrum
The Special Properties of Laser Light
Reading Assignment
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Please read the following from Serway
and Jewett before class on Monday:
Chapter 25, Sections 25.1 through 25.4.
A www.masteringphysics.com assignment is
due this Friday at 5:00 PM. It is based on
Chapter 14 material: interference,
standing waves and harmonics.
Note from last day’s lecture:
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For a standing wave of sound in an air
column, the pressure nodes correspond to
displacement antinodes. Conversely, the
pressure antinodes correspond to
displacement nodes.
It is the pressure variations which cause a
microphone or eardrum to vibrate. Therefore,
the standing wave sounds loudest near the
pressure antinodes, or displacement nodes.
(this can be discovered in the “Standing Waves
and Acoustic Resonance” Lab)
Pair 1
Pair 2
The traces above show beats that occur when two
different pairs of waves are added. For which of
the two is the difference in frequency of the
original waves greater?
1. Pair 1
2. Pair 2
3. The frequency difference is the same for both.
4. Need more information.
Pair 1
Pair 2
The traces above show beats that occur when two
different pairs of waves are added. For which of
the two is the sum in frequency of the original
waves greater?
1. Pair 1
2. Pair 2
3. The frequency sum is the same for both.
4. Need more information.
Which of these thingies are not the same?
Heat
Microwaves
Sound
X-rays
Radio
Light
Ultraviolet
Infrared
Everything you need to know about
Electricity and Magnetism (for now…)
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E

B
= Electric Field, which produces a force
on charged objects. Units are
Newtons/Coulomb, where “Coulomb” is
a measure of electric charge.
= Magnetic Field, which causes
compasses to point, and produces a
force on moving charged objects. Units
are called “Tesla”.
Results of Maxwell’s Equations:
 E 1  E

2
2
2
x
c t
The Wave Equation for
Electric Field.
 B 1  B
 2 2
2
x
c t
The Wave Equation for
Magnetic Field.
2
2
2
2
c is a new constant of the universe:
c = 2.998×108 metres/second.
Results of Maxwell’s Equations:
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EB
E
c
B
The relating equations
between Electric and Magnetic
fields in an electromagnetic
wave.
c is a constant of the universe:
c = 2.998×108 metres/second.
The Electromagnetic Spectrum
act more like waves
higher
frequency
Radio
Microwaves
Infrared Waves
Visible Light
Ultraviolet Light
X-rays
Gamma Rays
act more like particles
longer
wavelength
“Lasers”
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Light Amplified by Stimulated Emission
Radiation
Eye surgery: corneal transplants, vision
correction
Heart surgery
Laser imaging for diagnosis
Laser dentistry