1. Physics 208, Fall 2008
Waves: Interference and Diffraction (Chap )
Colors in soap films, CDs, and butterfly wings
Refraction of Light, and Optics (Chap )
Telescopes, microscopes, and eyes
Electricity (Chap )
Capacitors & cell membranes, battery-less flashlights
Magnetism (Chap )
Electromagnetic Waves (Chap. 35)
Light and polarization
Modern Physics (Chap )
Radiation and matter, radioactivity, nuclear fission
Tuesday, Sep. 2
Phy208 Lecture 1

2. Your Professor and TAs
TA consultation hours are in 2331 Chamberlin. Hours will be posted.
Tuesday, Sep. 2
Phy208 Lecture 1

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. 12 (next week)
Required for those signed up for honors (B or better course grade also required for honors)
Tuesday, Sep. 2
Phy208 Lecture 1

4. Discussions and Labs
Discussions Monday and Wednesday (begin tomorrow, Sep. 3)
Weekly Q&A, group problem, quiz.
Labs
- start next week (3254 Chamberlin).
- No lab notebook: question sheets distributed.
- 10 of 11 labs must be completed to pass the course.
- Complete only 10 -> 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.
Tuesday, Sep. 2
Phy208 Lecture 1

5. Text: (Same as Physics 207 last semester)
Homework: (Mastering Physics)
Should be able to use your login from Physics 207
Follow instructions for registering
Assigned each Thursday, due 8 days later Friday at midnight.
First assignment: HW1 assigned Thurs, Sep. 4 due Friday, Sep. 12 at midnight
Tuesday, Sep. 2
Phy208 Lecture 1

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: Mon. Sep :30-7 pm
Exam 2: Tue. Oct :30-7 pm
Exam 3: Tue. Nov :30-7 pm
Cumulative Final Exam: Mon. Dec 15, 10:05 am -12:05 pm
Exams make up most of your grade (70%)
Tuesday, Sep. 2
Phy208 Lecture 1

7. Grading Weight Comments Labs 10%
10 of the 11 labs required to pass the course Lab grade reduced 30% if only 10 labs completed
Discussion
Mon & Wed
Homework
Mastering Physics
Exams:
MTE1
MTE2
MTE3
Final
15 %
15%
25%
1:30 hrs
2:00 hrs
Tuesday, Sep. 2
Phy208 Lecture 1

8. 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
Tuesday, Sep. 2
Phy208 Lecture 1

9. Last semester: mechanical waves
This semester: electromagnetic waves
Examples
Sound waves
Water waves
Earthquake waves
Medium required
Wave describes displacement of medium
Examples
Light waves
Radio waves
Microwaves
X-rays
No medium required
Can propagate in vacuum
Wave describes local electric and magnetic fields
Tuesday, Sep. 2
Phy208 Lecture 1

10. Common wave properties
Reflection
Superposition
Waves have have more ‘properties’ than particles. Here the wave can be ‘positive’; or ‘negative’. Later we will call this phase.
Tuesday, Sep. 2
Phy208 Lecture 1

11. Wave properties Wave has both space and time dependence
Put card on rope halfway along room. Rope knocks off card -> shows that the moving rope has energy and can transmit some of that energy.
Shows constant time intervals
Tuesday, Sep. 2
Phy208 Lecture 1

12. 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
Explain on board position dependence at constant time, time dependence at constant position
trough
Mathematically:
Tuesday, Sep. 2
Phy208 Lecture 1

13. Moving wave crests Period = 1 second Propagation speed = v
Mark wavelength on the board for these two cases
Tuesday, Sep. 2
Phy208 Lecture 1

14. Same period, slower wave velocity
Period = 1 second
Propagation speed = v/2
Should change this to make it same frequency (time interval between pulses), but different propagation speeds.
Tuesday, Sep. 2
Phy208 Lecture 1

15. 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
Tuesday, Sep. 2
Phy208 Lecture 1

16. Wave relations Velocity = Wavelength / Period v =  / T, or v = f
f = Frequency = 1 / Period = 1/T
Sinusoidal traveling wave: or
Tuesday, Sep. 2
Phy208 Lecture 1

17. 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.
This is called interference
Tuesday, Sep. 2
Phy208 Lecture 1

18. Destructive Interference in a String
Two pulses are traveling in opposite directions
Net displacement = sum of individual displacements
Pulses unchanged after interference
Tuesday, Sep. 2
Phy208 Lecture 1

19. 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 + =
Tuesday, Sep. 2
Phy208 Lecture 1

20. 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
Tuesday, Sep. 2
Phy208 Lecture 1

21. 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)
0.1 m
0.25 m
0.5 m
1.0 m
2.0 m
Tuesday, Sep. 2
Phy208 Lecture 1

22. Interference of 2 speakers
constructive interference,loud tone
destructive interference quit tone
crest trough
Tuesday, Sep. 2
Phy208 Lecture 1

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
Tuesday, Sep. 2
Phy208 Lecture 1

24. Other angles? Path-length difference 0 Interference:
Constructive: =
Destructive: =
Path 1
Path 2
Do this part on board. For f = 2000 Hz, wavelength = 0.17 m. Speaker separation d=1m.
Want 0.17m=1m x sin(theta)->theta=9.87. This is angle between two maxima. 
 = Extra path length d
Tuesday, Sep. 2
Phy208 Lecture 1

25. Interference engineering
Tuesday, Sep. 2
Phy208 Lecture 1

26. Line array works by interference
Off-axis sound canceled by interference on the vertical axis.
Horizontal plane unaffected
Total sound intensity drops off more slowly
Tuesday, Sep. 2
Phy208 Lecture 1