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Physics 208, Spring 2009 http://www.physics.wisc.edu/undergrads/courses/spring09/208/index.html

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Physics 208, Topics

  Light Waves: Interference and Diffraction (Chap. 21-22)   Compact discs, and butterfly wings

  Refraction of Light, and Optics (Chap. 23-25)   Telescopes, microscopes, eyes…

  Electricity (Chap. 26-32)   Electric force, Circuits, nerves..

  Magnetism (Chap. 33-34)   Magnetic force, MRI...

  Electromagnetic waves (Chap. 35-36)   Radio, microwave, infrared, visible light, ultraviolet…

  Modern Physics (Chap. 37-42)   Radiation and matter, radioactivity, nuclear fission

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Your Professor and TAs

See course page for contact information and consultation hours and locations.

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Lectures   M,W 8:50-9:40, 2103 Chamberlin   Reviews reading

  Demonstrations   Examples   Quick questions   Attendance will be taken

  Honors lectures on select Fridays (see syllabus)   All students welcome. Required for those registered for

honors credit.

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Discussions and Labs Discussions (begin tomorrow, Jan 22)

Tuesday, weekly group problem Thursday, weekly quiz (5 problems) started and completed on web by Friday midnight.

Labs (start next week) No lab notebook. Question sheets distributed via course home

page. Reference lab manual available for download or purchase. 9 of 10 labs must be completed to pass the course. Complete only 9 -> lab grade reduced by 30%.

•  If you must miss a lab: •  Anticipate and immediately contact your TA •  Try to arrange for a different lab section that week •  If that is not possible, lab missed (for a good reason) can be

made up only during the exam week immediately following.

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Homework: (Mastering Physics)

Use your login from Physics 207

•  Weekly exercises and tutorials, 5 per chapter

•  Details on syllabus at at Mastering Physics website

•  Hints and partial credit available!

Text: (Same as Physics 207 last semester)

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Exams

Exam 1: Wed. Feb. 18 5:30-7 pm Exam 2: Wed. Mar. 25 5:30-7 pm Exam 3: Wed. Apr. 22 5:30-7 pm Cumulative Final Exam: Thurs. May 14, 2:45 -4:45 pm

Exams scores constitute most of your grade. No makeup of early exams will be available.

 Three midterm exams, 1 hr 30 min each  Typically 5-6 problems (old exams at web site)

  Multiple choice   Short answer   Longer calculation

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Physics Learning Center •  Structured small group sessions •  Review sessions •  Practice problems •  Focus on concepts •  Opportunity to meet study partners •  Room 2338 Chamberlin •  Contact Larry Watson

Room 2338 lwatson@physics.wisc.edu

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Last semester: mechanical waves   Examples

  Sound waves   Water waves   Seismic waves

  Medium required   Wave corresponds to

localized displacement of medium

This semester: electromagnetic waves   Examples

  Light waves   Radio waves   X-rays

  Through medium or vacuum   Wave corresponds to

localized variation in electric and magnetic force fields

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Common wave properties

Reflection Superposition

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Wave properties •  A wave has both space and time dependence

The magic: What moves is energy and momentum, the space through which the wave propagated returns to its quiescent state. Often internal internal energy loss in the medium is small. The wave motion appears frictionless.

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Harmonic wave

•  A harmonic mechanical wave is generated by harmonic source (e.g. a vibrating tuning fork) of period T and frequency f=1/T (hz).

•  The speed v is characteristic of the medium •  The wavelength (repeat distance) is the

distance the disturbance propagates in (repeat time) T. λ:= vT

 v

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Ultrasound A piezoelectric transducer

vibrating at 10 MHz generates a wave in human tissue with natural wave speed 1540 m/s. What is the wavelength of the sound wave?

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Ultrasound A piezoelectric transducer

vibrating at 10 MHz generates a wave in human tissue with natural wave speed 1540 m/s. What is the wavelength of the sound wave?

 λ= vT =v/f = (1540 m/s)/1e7(1/s)  = 1.54e-4 m = 154 micron

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Interference

•  Overlapping wave disturbances are additive. (Principle of superposition)

•  In some areas they cancel, in others they reinforce.

•  This is called interference.

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Interference of wave pulses

•  Two pulses travel in opposite directions

•  Net displacement = sum of individual displacements

•  Where pulses overlap, interference can be destructive (as shown for pulses of opposite sign) or constructive

•  Pulses unchanged after interference

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Interference of harmonic waves

•  Two waves, a and b, have the same amplitude and frequency

•  When they are 1/2 wavelength out of phase (peak matches trough) the waveforms cancel everywhere.

•  Conversely, if in phase the net wave has twice the amplitude.

+

=

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Interference of harmonic waves: opposite directions

•  Two similar waves moving in opposite directions go in and out of phase periodically forming a periodic standing wave.

•  Standing waves are formed in bounded spaces through the superposition of incident and reflected waves

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Interference of harmonic waves: same direction

•  Two similar waves moving in the same direction form a traveling interference pattern.

•  Interference is constructive if the waves are in phase and the total wave amplitude is double.

•  Interference is destructive if the waves are out of phase and the wave amplitude is zero.

Noise cancelling headphones

•  Active sonic noise cancellation is a commercial success.

•  A microphone detects incoming sound wave. A speaker creates a cancelling sound, a copy of the incident wave with opposite amplitude.

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+

=

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Setting up destructive interference •  Sound takes two paths to arrive at ear •  One path 1/2 wavelength longer than other •  Results in destructive interference

Destructive

Destructive interference for frequencies such that path length difference is 1/2 wavelength.

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Question Two speakers emit similar 340 Hz sound waves in

the same direction. At what speaker separation x would the sound from two speakers cancel? (sound velocity = 340 m/s)

A.  0.1 m

B.  0.25 m C.  0.5 m D.  1.0 m E.  2.0 m

 x

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Question Two speakers each emit a 340 Hz sound wave. At

what speaker separation would the sound from two speakers cancel? (sound velocity = 340 m/s)

A.  0.1 m

B.  0.25 m C.  0.5 m D.  1.0 m E.  2.0 m  Wavelength = 340 (m/s)/340 (1/s)=1 m

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Two dimensional interference

•  Example 21.12 To find the amplitude at a point, find the path lengths and phases of the two waves at that point then apply superposition.

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Interference pattern

Light waves

•  Light is a wave carrying electromagnetic energy and can propagate through “empty space.”

•  Evidence: Interference

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Closing thought

•  Light is a wave. •  Chapter 21 discusses an optical application

of interference – the antireflection coating on glasses.

•  Is it possible to produce an optical analog of sonic noise cancellation in the form of an active cloak and render oneself invisible?

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