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Physics 208, Fall 2009

Physics 208, Fall 2009. www.physics.wisc.edu/undergrads/courses/fall09/208. Waves: Interference and Diffraction (Chap. 20-22) Colors in soap films, CDs, and butterfly wings Refraction of Light, and Optics (Chap. 23-24) Telescopes, microscopes, and eyes Electricity (Chap. 26-32)

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Physics 208, Fall 2009

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  1. Physics 208, Fall 2009 www.physics.wisc.edu/undergrads/courses/fall09/208 • Waves: Interference and Diffraction (Chap. 20-22) • Colors in soap films, CDs, and butterfly wings • Refraction of Light, and Optics (Chap. 23-24) • Telescopes, microscopes, and eyes • Electricity (Chap. 26-32) • Capacitors & cell membranes, battery-less flashlights • Magnetism (Chap. 33-34) • Electromagnetic Waves (Chap. 35) • Light and polarization • Modern Physics (Chap. 37-42) • Radiation and matter, radioactivity, nuclear fission Phy208 Lecture 2

  2. Your Professor and TAs TA consultation hours are in 2331 Chamberlin. Hours will be posted. Phy208 Lecture 2

  3. Lectures • Tuesday, Thursday 12:05-12:55 2103 Chamberlin • Includes • Demonstrations • Examples • Quick questions • Honors section • Friday 12:05-12:55, 2103 Chamberlin • First meeting Sep. 11 (next week) • Required for those signed up for honors(B or better course grade also required for honors) Phy208 Lecture 2

  4. Discussions and Labs Discussions Monday and Wednesday (begin Wed, Sep. 9) Weekly Q&A, group problem, quiz. Labs - start week of Sep 14 (3254 Chamberlin). - No lab notebook: question sheets distributed. - 8 of 9 labs must be completed to pass the course. - Complete only 8 -> lab grade reduced by 30%. • If you must miss a lab: • Immediately contact your TA • Try to arrange for a different lab section that week • If not possible, lab missed (for a good reason) can be made up only during the immediately following exam week. Phy208 Lecture 2

  5. Text: (Same as Physics 207 last semester) Homework: (Mastering Physics) Should be able to use your login from Physics 207 • Follow email instructions for registering (as soon as possible) • Course ID: PHYSICS208FALL2009 • Assigned each Thursday – Week2HW available now. Due 8 days later Friday at midnight. Phy208 Lecture 2

  6. Exams • Three midterm exams, 1 hr 30 min each • Typically 5-6 problems (old exams at web site) • Multiple choice • Short answer • Longer calculation Exam 1: Tue. Sep. 29 5:30-7 pm Exam 2: Tue. Oct. 27 5:30-7 pm Exam 3: Thur. Dec. 3 5:30-7 pm Cumulative Final Exam: Mon. Dec 21, 5:05 pm -7:05 pm Exams make up most of your grade (70%) Phy208 Lecture 2

  7. Grading Phy208 Lecture 2

  8. This semester: electromagnetic waves Last semester: mechanical waves • Examples • Light waves • Radio waves • Microwaves • X-rays • No medium required • Can propagate in vacuum • Wave describes local electric and magnetic fields • Examples • Sound waves • Water waves • Earthquake waves • Medium required • Wave describes displacement of medium Phy208 Lecture 2

  9. Common wave properties Reflection Superposition Phy208 Lecture 2

  10. Wave properties • Wave has both space and time dependence Shows constant time intervals Phy208 Lecture 2

  11. Mathematically: Periodic wave • Repeats continuously, moves in space • Wavelength : distance to complete one cycle • Period T: time to complete one cycle • Frequency f: 1/Period • crest • trough Phy208 Lecture 2

  12. Moving wave crests • Period = 1 second • Propagation speed = v Phy208 Lecture 2

  13. Same period, slower wave velocity • Period = 1 second • Propagation speed = v/2 Phy208 Lecture 2

  14. Freq., wavelength, velocity are related • Source emits a crest once every period T. • This crest propagates at velocity v. • By the time the source emits another crest T seconds later, the first crest has moved vT. • So the distance between crests is vT. • This is the wavelength of the wave • Wavelength = velocity  period Phy208 Lecture 2

  15. Wave relations • Velocity = Wavelength / Period • v =  / T, or v = f • f = Frequency = 1 / Period = 1/T • Sinusoidal traveling wave: • or Wavenumber( m-1 ) Angular frequency(rad/s) Phy208 Lecture 2

  16. Phase difference between waves • Waves of same amplitude, wavelength, frequency, can still be “different”. • Can be phase difference between waves • λ Here the phase difference is • 180˚ • π radians • “1/2 wavelength” Phase difference Phy208 Lecture 2

  17. Mathematical expression for phase • Phase is the argument of the sin function • Phase shift is extra displacement of wave along propagation direction. • Rigidly moves the positions of the crests, troughs, and zeroes Wavenumber Angular frequency Phase Shift Phy208 Lecture 2

  18. Interference • Water drop is a source of circular waves (two-dimensions here) • When the waves overlap, they superimpose. • In some areas they cancel, in others they reinforce. • Depends on phase difference. Phy208 Lecture 2

  19. Destructive Interference in a String • Two pulses are traveling in opposite directions • Net displacement = sum of individual displacements • Pulses unchanged after interference Phy208 Lecture 2

  20. Destructive interference in a continuous wave • Two waves, a and b, have the same amplitude and frequency • They are 1/2 wavelength out of phase • When they combine, the waveforms cancel + = Phy208 Lecture 2

  21. 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 interference for frequencies such that path length difference is 1/2 wavelength. Destructive Phy208 Lecture 2

  22. constructive interference,loud tone destructive interferencequit tone Interference of 2 speakers cresttrough Phy208 Lecture 2

  23. A little more detail Path-length difference  = (Path 2) - (Path 1) = 0 Same distance, same phase Path 1 Path 2 Path 1 Path 2 d Phy208 Lecture 2

  24. Other angles? Path-length difference 0 Interference: Constructive: = Destructive: = Path 1 Path 2   = Extra path length d Phy208 Lecture 2

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