1. Physics 7C SS1 Lecture 3: Waves
Quiz 1
Understanding Interference

2. Reviewing the Basics Constructive Interference:
total phase equivalent
Destructive Interference:
total phase separated by half cycle
Note: these waves have identical periods and wavelengths, but that is not always the case!
Show webpage; show policy.
Fee waiver: may seek a waiver from the department (forms available with me in room 225 Phy/Geo) *before the 20th day of instruction.*
Extra time required on exams? Let me know after class.

3. Checking 1D interference
Two speakers are connected to the same function generator with reversed wiring. One speaker is placed a half wavelength in front. What type of interference results?
A) Constructive
B) Destructive
C) Partial
D) Depends on what time you choose
E) Insufficient information to decide

4. Checking 1D interference
Two speakers are wired to the same function generator. The phase constants differ by /2. One speaker is placed a quarter wavelength in front. What type of interference results?
A) Constructive
B) Destructive
C) Partial
D) Depends on what time you choose
E) Insufficient information to decide

5. Checking 1D interference
Two speakers are wired to the same function generator. The phase constants differ by /2. One speaker is placed a quarter wavelength in front. What type of interference results?
A) Constructive
B) Destructive
C) Partial
D) Depends on what time you choose
E) Insufficient information to decide

6. Checking 1D interference
Two speakers are wired into different function generators with slightly different frequencies. They have identical wiring, and are placed face-to-face. What type of interference results?
A) Constructive
B) Destructive
C) Partial
D) Depends on what time you choose
E) Insufficient information to decide

7. Interference for differing frequencies: Beats.1 2 3 4 5
What type of interference occurs at each point?

8. Interference for differing frequencies: Beats1 2 3 4 5

9. Interference for differing frequencies: Beats1 2 3 4 5
The carrier frequency
The beat frequency

10. Interference for differing frequencies: Beats
What’s happening at the gold arrows?
What’s happening at the black arrows?

11. Checking 1D interference
Two speakers are wired into different function generators with slightly different frequencies. They have identical wiring, and are placed face-to-face. What type of interference results?
A) Constructive
B) Destructive
C) Partial
D) Depends on what time you choose
E) Insufficient information to decide

12. Introducing 2D Interference: The Ripple Tank
Try to spot…
Places that are very deep
Places that are very shallow
Places that are “normal” depth
Places two crest combine
Places two troughs combine
Places a crest meets a trough

13. Two slits create two overlapping wave frontsd
Crest
Trough

14. 2D Interference Conceptually, nothing new!
Temporal contribution if frequencies differ
Typically same medium & same source
Pathlength difference contribution
How far has each wave traveled?
Phase constant contribution
By how much do phases differ? (Typically same phase difference, though not always!)
Summary: Just like 1D interference, but typically you only need to think about pathlength difference!

15. Two slits create two overlapping wave frontsd
Crest
Trough

16. Two slits create two overlapping wave frontsd
What type of interference occurs at the marked spot?
Constructive
Destructive
Partial
Time-dependent
Crest
Trough

17. Two slits create two overlapping wave frontsd
Why?
x=0
x=/2
x
None of the above
Crest
Trough

18. Two slits create two overlapping wave frontsx1 x2
Why?
x=0
x=/2
x
None of the above
x1=x2 so x=0!
Crest
Trough

19. Two slits create two overlapping wave frontsd
What type of interference occurs at the marked spot?
Constructive
Destructive
Partial
Time-dependent
Crest
Trough

20. Two slits create two overlapping wave frontsd
Why?
x=0
x=/2
x
None of the above
Crest
Trough

21. Two slits create two overlapping wave frontsd
Why?
x=0
x=/2
x
None of the above
Crest
Trough

22. Two slits create two overlapping wave frontsd
What type of interference occurs at the marked spot?
Constructive
Destructive
Partial
Time-dependent
Crest
Trough

23. Two slits create two overlapping wave frontsd
Why?
x=0
x=/2
x
None of the above
Crest
Trough

24. Two slits create two overlapping wave frontsd x2 x1
Why?
x=0
x=/2
x
None of the above
x1>x2 now…
x1=2
x2=1.5
Crest
Trough

25. An interesting phenomenon
What are you hearing? Listen carefully
A) Constructive Interference Only
B) Destructive Interference Only
C) Beats
D) Pathlength-dependent Interference: both constructive & destructive depending on position
E) Partial Interference
This is when there were two function generators. You could not hear sound if only one were connected. You could hear sound if both were connected.

26. An interesting phenomenon
What are you hearing? Listen carefully
A) Constructive Interference Only
B) Destructive Interference Only
C) Beats
D) Pathlength-dependent Interference: both constructive & destructive depending on position
E) Partial Interference
You heard the beat frequency. Each speaker’s frequency was above 20,000Hz and outside your hearing range. The carrier frequency was also outside your hearing range.

27. Laser Interference
In DL you will use a shortcut to calculate pathlength difference: d sin = x

28. Quiz 1