1. Waves
AP Physics

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Definitions
Wave – A disturbance in a given medium that carries energy from one place to another
Medium – substance through which a wave travels; not all ways need mediums (plural: media)
Pulse Wave – A single disturbance in a medium
Continuous Wave – A set of disturbances occurring over and over usually in a pattern

3. What properties help describe a wave?
Amplitude:
the distance from the midpoint to the crest (or trough) of the wave.
is the maximum displacement from equilibrium

4. Types of Waves Longitudinal Waves (Compressional)
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Types of Waves Longitudinal Waves (Compressional)
Type of wave in which the medium vibrates in the same direction as the movement of energy
Wave consists of a series of compressions and rarefactions
Example: Sound

6. Types of Waves Transverse Waves
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Types of Waves Transverse Waves
Type of wave in which the medium vibrates perpendicular to the movement of energy
Wave consists of a series of crests and troughs
Example: Light

8. Transverse vs. Longitudinal Waves

9. Types of Wave Surface Waves
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Types of Wave Surface Waves
Type of wave in which the medium vibrates in a circular motion; a combination of up to down and side to side
Example: Water waves

11. Wave Properties Wavelength – l (lambda)
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Wave Properties Wavelength – l (lambda)
Distance in a wave between two like points, i.e. distance from compression to compression or from crest to crest
Wavelength
NOT Wavelength
Wavelength

12. Wave Properties Period and Frequency
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Wave Properties Period and Frequency
Period (T) – the amount of time it takes for one wave to pass
Frequency ( f ) – the number of waves that pass in a second
For Example…

13. 15 Wavelengths in 5.0 sec. Time = 1.5 sec Time = 1.0 sec14 4 5 3 2 1 13 6 11 12 7 10 9 8
Wavelengths
15 Wavelengths in 5.0 sec.

14. Period = = = Frequency = = = = Period and Frequency Time 5.0 sec
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Period and Frequency
Time
5.0 sec
Period = =
# of Waves
15 waves =
0.33 sec
# of Waves
15 waves
Frequency = =
5.0 sec
Time = =
3.0 1/sec
3 Hz

15. Wave Properties Period and Frequency
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Wave Properties Period and Frequency
Frequency is the inverse of Period f = 1/T The unit for frequency is 1/sec or Hz (Hertz)

16. Wave Properties Speed of a Wave
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Wave Properties Speed of a Wave
We know that
Speed = how far / period of time.
A wave will travel one wavelength in one period of time
So, v = l/T
Since f = 1/T, then …
v = lf
BTW, in college, you may see the Greek letter nu (n) used for frequency

17. Wave Properties Speed of a Wave (cont.)
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Wave Properties Speed of a Wave (cont.)
The speed of a wave DOES NOT change unless the medium changes.
i.e., for sound, temperature and pressure can change the speed of sound.
The electro-magnetic fields can not be changed; so the speed of light will be constant.
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18. Wave Properties Speed of a Wave (cont.)
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Wave Properties Speed of a Wave (cont.)
Assuming a medium doesn’t change, what has to happen to the wavelength of a wave if you increase the frequency?
Wavelength will decrease
What has to happen to the frequency of a wave if you shorten the wavelength?
Frequency will increase
v = lf

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Example #1
In music, the note B has a f = 247 Hz and a l = 1.34 m. What is the speed of these waves in air (a.k.a. sound)?
v = lf = (1.34 m)(247 Hz)
= 331 m*Hz = 331 m/s

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Example#2
Suppose a planet is emitting a red light. Red light is known to have a wavelength of 7.00 x 10-7 m. Knowing the speed of light, what is the frequency of the light?
f = v/l = (3.00 x 108 m/s)/(7.00 x 10-7 m)
= 4.29 x 1014 Hz

21. Example #2 (cont.) For the previous example,
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Example #2 (cont.)
For the previous example,
f = 4.29 x 1014 Hz. What is the period of the red light?
𝐓 = 𝟏 𝒇 = 𝟏 𝟒.𝟐𝟗× 𝟏𝟎 𝟏𝟒 𝐇𝐳
= 2.33 x sec

22. Wave Behaviors Superposition of Waves
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Wave Behaviors Superposition of Waves
When two objects meet, they cannot occupy the same time and space; however, waves can pass through each other

23. Wave Behaviors Interference
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Wave Behaviors Interference
If two waves are on the same side of the rest position, then the two waves will have constructive interference (add to each other)

24. Wave Behaviors Interference
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Wave Behaviors Interference
If two waves are on opposite sides of the rest position, then the two waves will have destructive interference (subtract from each other)

25. Constructive Interference
In constructive interference, the waves reinforce each other to produce a wave of increased amplitude.

26. Constructive Interference

27. Destructive Interference
In destructive interference, the waves cancel each other and no wave is produced.

28. Wave Interference

29. Wave interference demo

30. Interference (different speed; same wavelength)

31. Interference – Beats (same speed; different wavelength)

32. Wave Behaviors Standing Waves
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Wave Behaviors Standing Waves
A continuous wave that has points of no movement (nodes) and maximum movement (antinodes)
Combination of reflections and interference

33. Measuring Speed Using Standing Waves
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Measuring Speed Using Standing Waves
Measure Wavelength
Count number of crests in a given time period; time for 10 crests?
Divide 10 by that time to get frequency
v = fl