Waves

1
Waves
2
What is a Wave?

• A wave is an energy vibration that usually takes
  place in matter
• Key word vibration. Waves repeat
• A single wave is called a pulse

3
Wave Types

• Transverse Waves vibrate at a right angle to
  their direction of motion
• Examples Light, waves on a rope, sine waves

4

• Longitudinal Waves vibrate in the direction of
  motion
• Examples Sound, P-Waves, Slinky Waves

5
Surface Waves

• Surface waves are exactly what they sound like
  waves that travel on a surface (a boundary
  between two different forms of matter)
• Some look like Transverse waves (ocean waves)
• Others move like Longitudinal waves along a
  surface (Rayleigh waves)

6
(No Transcript)
7
Other Wave Types

• Bow Waves
• Shock Waves

8
(No Transcript)
9

• http//www.carteretcountyschools.org/chs/fptwebs/J
  MILBOURNE/Physics/video20clips/mach1.mpg

10
(No Transcript)
11
Wave Properties

• We use four different characteristics to talk
  about waves frequency, wavelength, amplitude,
  period, and speed
• Wavelength is the distance between identical
  parts of a wave (crest?crest, trough?trough)
• Amplitude is half the waves total height, or the
  distance between the crest and the rest position

12
(No Transcript)
13
Some Wavelengths

• Microwaves .03 m
• Radio Waves 10 m
• Ocean Waves 200 m
• Tsunami 11,000 m
• 200,000 m

14
Wave Time

• A waves period is the time it takes for one wave
  to pass through a given point (time a buoy w/ a
  watch)
• If you divide wavelength by time, you get a
  waves speed (speed distance/time)

15
Some Wave Speeds

• Ocean Waves (on shore) 4 m/s
• Tsunami Waves 30 m/s
• Sound (in air) 340 m/s
• P Waves 1500 m/s
• Light (in air) 300,000,000 m/s

16
Frequency

• A waves frequency tells us how often waves occur
  (2 waves a second, 4 waves a second, etc)
• The units of frequency are Hertz, or Hz
• 1 Hz 1 wave/second
• Frequency 1/period

17
How does frequency change?
18

• Draw a wave with ½ the frequency and twice the
  amplitude of the wave below

19
Wave Speed, Remixed

• Speed wavelength/period
• If period 1/frequency, then.
• Speed wavelength x frequency
• v lf

20
Problems

• A rope is vibrated to produce a wave with
  wavelength .25m. On the rope, 4 waves occur
  every 2 seconds. What is the speed of this wave?
  
• An ocean wave has a period of 15s and moves at
  4m/s. What is its wavelength?

21
Matter Matters

• In a given medium, wave speed is constant (sound
  always has the same speed in air)
• However, if you change media the waves speed
  changes
• Sound travels faster in water than in air, faster
  in steel than in water
• Why?

22

• Why did folks in the old days put their head
  next to railroad tracks?

23
Wave Behavior

• All waves do four interesting things when they
  interact with matter
• They reflect, refract, interfere, and diffract

24
Reflection

• Reflection occurs when a wave hits a boundary,
  then goes in the opposite direction
• http//www.kettering.edu/drussell/Demos/reflect/r
  eflect.html

25
(No Transcript)
26
Interesting Consequences

• Echos, mirrors, standing waves

27
Echoes

• A person shouts into the grand canyon

28
Interference

• When waves interact with each other, they combine
• Interference is just a fancy way of saying that
  waves add together
• http//www.kettering.edu/drussell/Demos/superposi
  tion/superposition.html

29
(No Transcript)
30
Concert Seats?

• When buying tickets to a concert, you might want
  to think about interference
• How might sound volume change in places with
  different types of interference?
• A buildings acoustics are related to how waves
  combine in different locations

31
(No Transcript)
32
Speakers

• In a similar way, speaker placement for a
  surround sound system depends on interference

33
Standing Wave Patterns

• As weve seen, wave reflection and interference
  result in standing waves
• Standing wave patterns depend upon the string
  length, wave speed, and frequency of vibration

34

• http//physics.usask.ca/hirose/ep225/animation/st
  anding1/anim-stwave1.htm

35
Creating Standing Waves

• Imagine you have a string of length L, which you
  can shake with your hand
• Sketch all the standing wave patterns you can
  imagine on this string

36
(No Transcript)
37
Nodes and Antinodes

• Nodes points on the wave where the wave
  amplitude 0
• Antinodes points of maximum/minimum amplitude

38
Harmonics

• As you can see from the previous slide, one can
  create a variety of standing wave patterns on a
  given string
• The first standing wave pattern (SWP) is called
  the fundamental, first harmonic, or first order
• The second SWP is called the second harmonic or
  second order
• The third SWP is called the third harmonic or
  third order

39
Wavelength Generalizations

• Draw the first four SWP for the string of length
  L
• Write down the wavelength of each, in terms of L
• l1 2L l2 L 2L/2
• l3 2L/3 l2 L/2 2L/4
• Notice a pattern?

40

• lN 2L/N
• N harmonic or order number
• If v lNfN (2L/N)fN
• Then the generic frequency fN Nv/2L

41
Other SWP

• How about a standing wave on a string with one
  end that isnt fixed?
• Draw the first three patterns for this type of
  wave and see if you can find the generic
  wavelength
• http//mysite.verizon.net/vzeoacw1/harmonics.html

42
(No Transcript)
43

• lN 4L/(2N 1)
• fN (2N-1)v/4L

44
Two Open Ends?

• How about a string with two ends that can move?

45
(No Transcript)
46

• lN 2L/N
• N harmonic or order number
• If v lNfN (2L/N)fN
• Then the generic frequency fN Nv/2L

47
Wave Questions
48
Basics of Sound
49
(No Transcript)
50
Some Sound Facts
How much denser are the high pressure regions?
For a rock concert, about 141 more molecules for
every 1 million!
51
The Vibrations of Speech
Primary Vibrations
Secondary Vibrations???
52
Sound Detection
Hair Cells of the inner ear
53
Range of Human Hearing
54
Resonance

• Every object has a so-called natural frequency
  (swing sets)
• If you hit a tuning fork, it always vibrates at
  the same frequency
• When something vibrates at its natural frequency,
  we call it resonance

55
(No Transcript)
56
Raising the stakes

• If you continue vibrating something at its
  resonance frequency, the amplitude of the
  vibration increases

57
Shattering a Wine Glass?
58
Doppler Effect Moving Sounds

• Up to this point, weve been talking about sound
  sources that dont move
• The question is, what happens when these sources
  do move?
• Think about how a fire truck sounds as it moves
  down the road?

59
Shorter Ahead, Longer Behind
60
(No Transcript)
61
Doppler Radar
62
Beats

• How do musicians tune their instruments?

63
Diffraction

• Diffraction is similar to refraction, as it
  involves waves bending
• Diffraction occurs when waves bend as they
  encounter a boundary

64

• http//www.launc.tased.edu.au/online/sciences/phys
  ics/diffrac.html

65
Refraction

• When waves pass from one media to another at an
  angle, they refract, or bend
• Why? Because one side of the wave slows down

66

• http//www.launc.tased.edu.au/online/sciences/phys
  ics/refrac.html

67
ROYGBIV

• The best example of refraction is the prism,
  which separates white light into its component
  colors by bending them

68
Optical Illusions Mirages
69

• Light coming from the sky hits the ground, which
  is warmer than the surrounding air
• The light bends as it enters this warm air
• We always perceive light as traveling in a
  straight line, so we see the sky straight ahead
  of us, on the ground

70
(No Transcript)
71
Why is the sky blue?

• The air particles in our atmosphere refract blue
  light, scattering it around
• This makes it look like blue light comes from
  everywhere

72
(No Transcript)
73
Red Sunsets