PHY138

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PHY138 Waves, Lecture 6Todays overview

• Waves in the Electric and Magnetic Fields
• The Electromagnetic Spectrum
• The Special Properties of Laser Light

2
Reading Assignment

• 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 500 PM. It is based on Chapter 14
  material interference, standing waves and
  harmonics.

3
Note from last days lecture

• 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)

4
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.
5
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.
6
Which of these thingies are not the same?
Heat
Microwaves
Sound
X-rays
Radio
Light
Electromagnetic Waves
Ultraviolet
Infrared
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Everything you need to know about Electricity and
Magnetism (for now)
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.
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Results of Maxwells Equations
The Wave Equation for Electric Field.
The Wave Equation for Magnetic Field.
c is a new constant of the universe c
2.998108 metres/second.
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Results of Maxwells Equations
The relating equations between Electric and
Magnetic fields in an electromagnetic wave.
c is a constant of the universe c 2.998108
metres/second.
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The Electromagnetic Spectrum
act more like waves

• Radio
• Microwaves
• Infrared Waves
• Visible Light
• Ultraviolet Light
• X-rays
• Gamma Rays

longer wavelength
higher frequency
act more like particles
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Lasers

• Light Amplified by Stimulated Emission Radiation
• Eye surgery corneal transplants, vision
  correction
• Heart surgery
• Laser imaging for diagnosis
• Laser dentistry