PH 105

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PH 105
Wave Phenomena Interference and Diffraction
Dr. James van Howe Lecture 7
Water wave diffraction Sitges, Barcelona
Photo Barcelona Field Studies Center
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Announcements

• Exam 1 this Friday!!!
• Cover up through interference and diffraction
  (todays material)
• Multiple choice, short answer, calculation
• I will provide any equations you may need (no
  need to memorize equations but do need to
  understand how to use them)
• Review on Wednesday in class
• No make-ups except for excused absences (by Dean
  of Students)

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Outline

• Doppler Effect
• Interference
• Constructive
• Destructive
• Beats
• Diffraction
• Huygens-Fresnel Principle
• Barrier
• Single and double slit
• Poissons Spot

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Name the Artist/Composer

• Irving Berlin
• Maurice Ravel
• Jeff Buckley
• Rufus Wainwright

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Variation on an old one
Letting go of the mass will give oscillations at
a frequency of
k
Stretched an amount
m
To get half of the frequency, what needs to
happen to the mass?

• The mass should be doubled
• The mass should be cut in half
• The mass should be increased by four times
• The mass should be decreased by four times

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Refraction
Incoming sound wave
CO2
air
Faster velocity
?
What happens to the sound wave at the boundary?

• It bends toward the normal and the wavelength
  gets longer
• It bends towards the normal and the wavelength
  gets shorter
• It bends away from the normal and the wavelength
  gets longer
• It bends away from the normal and the wavelength
  gets shorter

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Diffraction
If a sound wave hits a door opening, the sound
will bent/spread
Spreading will be the most for
A.
B.
C.
D. None, they spread the same in each
case
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Doppler Effect
A change in the observed frequency of a wave due
to a moving source or a moving observer
Stationary Source
Longer wavelength
Shorter wavelength
Moving Source
Higher Frequency
Lower Frequency
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Doppler Effect Moving Observer
Stationary Source
Observer moves away source
Observer moves towards source
Counts less wavefronts in a given time
Counts more wavefronts in a given time
Frequency goes down
Frequency goes up
Use top signs for towards each other Use bottom
signs for away
Equation for all cases
New frequency
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Doppler Effect for Light
Earth
Red Shift
Blue Shift
Moving Galaxy
For light lower frequencies are red, and higher
frequencies are blue
The color of a moving galaxy gives a measurement
of velocity, which can give us a measurement of
distance of the galaxy
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Interference The property of waves to add up
(superpose) constructively or destructively
DP
Pressure, DP
position, x
Pressure Change
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Constructive Interference
Two speakers facing each other play the same
note, in phase, at the same amplitude
DP
position, x
Pressure, DP
Pressure Change
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Destructive Interference
Two speakers facing each other play the same
note, at the same amplitude, but 180 degrees out
of phase
DP
position, x
Pressure, DP
Pressure Change
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Examples of intentional acoustical destructive
interference

• Muffler in car/resonator
• Pipe length chosen so that reflections interfere
  with noise 180 degrees out of phase
• Noise canceling headphones
• Records noise and plays it back 180 degrees out
  of phase for cancellation

Demo video destructive interference
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Beats
The same speaker plays two tones with slightly
different frequency
DP
Pressure, DP
time, t
Pressure Change
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Beats
In Phase
In Phase
In Phase
Out of Phase
Out of Phase
DP
Pressure, DP
time, t
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Beats
DP
Pressure, DP
time, t
Envelope
If and ,
we hear a sine wave at
whose volume goes up and down (modulated) at

Envelope slow variation
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Diffraction
Diffraction Bending of waves from a barrier,
obstruction, or aperture
Really just interference of many spherical waves
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Huygens-Fresnel Principle
Every point on a wavefront serves as a secondary
source of point sources

Plane Wave
Infinite amount of point sources plane wave
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Waves will diffract around a barrier
Bending
Many point sources, but not infinite
Plane Wave
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Diffraction by a Single Slit
Case 1
Infinite point sources fit inside aperture
Shadow
plane wave out
Bright
Like light going through a window
Shadow
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Diffraction by a Single Slit
Many point sources, but not infinite
spreading
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Diffraction by a Single Slit
Slit so small only one point source fits
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Double Slit Diffraction
like two point sources
could be two speakers
Lines of constructive interference
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Poissons Spot
If obstruction size is on the order of
wavelength diffraction
Bright spot behind obstacle
Demo Video ripple tank, acoustic Poisson
Demo optics card
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Homework

• Ch3 R15, E2

Due at the beginning of class on Wednesday
R Review Question E Exercise