Physics 123C Waves

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Physics 123C Waves
Review 1Chapters 14, 20, and 21 April 13, 2005

• John G. Cramer
• Professor of Physics
• B451 PAB
• cramer_at_phys.washington.edu

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Review 1 Announcements

• Register your Clicker using the link on the
  123C Syllabus page as soon as possible.
• Lecture Homework 2 has been posted on the
  Tycho system. It is due at 900 PM tonight.
• Lecture Homework 3 will be posted soon on the
  Tycho system. It is due at 900 PM on
  Wednesday, April 27.
• On Friday, April 15 we will have Exam 1,
  covering Knight, Chapters 14, 20, and 21 (except
  20.7,8 and 21.8). You should bring with you to
  the Exam a Scantron sheet, a good calculator, and
  one sheet of 8.5x11 paper containing your notes
  on both sides.
• Before you come to the Exam on Friday, check
  for seat assignments on Tycho. Special requests
  for seats (left handed, aisle, front, back)
  should be sent to me by E-mail.

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Lecture Schedule (Weeks 1-3)
We are here
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Interference in 2D and 3D
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Path Length Difference
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Clicker Question 2
What type of interference occurs at point C?

• Maximum constructive
• Constructive, but less than maximum
• Perfect destructive
• Destructive, but not perfect
• There is no interference at point C.

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Nodes and Antinodes
We can construct surfaces defined by the
nodes and antinodes of 3D sources of spherical
waves. The shapes of the surfaces are hyperbolic
cylinders centered on the line connecting the two
sources. A 2D slice as in the figure shows
the nodal lines (cancellation) in black and the
antinodal lines (reinforcement) in red, under the
assumption that the waves are emitted with a
relative phase of zero, i.e., in phase.
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Example 2D Interference between Loudspeakers
Two loudspeakers are 2.0 m apart and produce
700 Hz sound waves in phase, in a room where the
speed of sound is 341 m/s. A listener stands 5.0
m from one loudspeaker and 2.0 m to one side of
center. Is the interference there
constructive, destructive, or something in
between? How will this result differ if the
speakers 1800 are out of phase?
Destructive interference if f00 Constructive
interference if f0p.
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Visualizing Interference
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Example Intensity of Two Interfering
Loudspeakers
Two loudspeakers are 6.0 m apart and produce
in-phase equal amplitude sound waves with a
wavelength of 1.0 m. Each speaker alone produces
an intensity I0. An observer at point A is 10.0
m in front of the plane containing the speakers
on their line of symmetry. A second observer at
point B is 10.0 m directly in front of one of the
speakers. In terms of I0, what is the
intensity IA at point A and IB at point B?
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Clicker Question 3
Two loudspeakers emit equal-amplitude sound
waves in phase with a wavelength of 1.0 m.
At the point indicated, the interference is

• Constructive
• Perfect destructive
• Something in between
• Cannot tell without knowing the speaker
  separation.

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Chapter 14 Summary (1)
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Chapter 14 Summary (2)
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Chapter 14 Summary (3)
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Damped Oscillations
Drag force
Dampingfactor
Sinusoidal variation
Ampl.
Note that damping reduces the oscillation
frequency.
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Chapter 20 - Summary (1)
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Chapter 20 - Summary (2)
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Chapter 20 - Summary (3)
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Chapter 21 - Summary (1)
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The Principle of Superposition
The principle of superposition When two or
more waves are simultaneously present at a single
point in space, the displacement of the medium at
that point is the sum of the displacements of
each individual wave. In other words, wave
displacements add linearly, and essentially
ignore each other.
We note that if the total displacement Dnet
becomes too large, superposition could fail and
the wave could go non-linear and flat-top.
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Chapter 21 - Summary (2)
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Chapter 21 - Summary (3)
Guitar, piano, violin
Open organ pipe
Closed organ pipe, Clarinet.
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Pipes and Modes
Open-Open or Closed-Closed
Open-Closed
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Two Layer Optics
Assume a phase flip of p at both interfaces.
Then the phase difference between the reflected
waves is
Note Notice that the index of refraction n
multiplies the path length d taken by the waves
reflected at the 2nd surface. This quantity nd
is often called the optical path length because
the path length is effectively lengthened by the
refractive medium.
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End of Review 1

• Register your Clicker using the link on the
  123C Syllabus page as soon as possible.
• Lecture Homework 2 has been posted on the
  Tycho system. It is due at 900 PM tonight.
• Lecture Homework 3 will be posted soon on the
  Tycho system. It is due at 900 PM on
  Wednesday, April 27.
• On Friday, April 15 we will have Exam 1,
  covering Knight, Chapters 14, 20, and 21 (except
  20.7,8 and 21.8). You should bring with you to
  the Exam a Scantron sheet, a good calculator, and
  one sheet of 8.5x11 paper with notes on both
  sides.
• Before you come to the Exam on Friday, check
  for seat assignments on Tycho. Special requests
  for seats (left handed, aisle, front, back)
  should be sent to me by E-mail.