1. Grade 11 Physics
Waves

2. The Nature of Waves A wave is a travelling disturbance
A wave carries energy from place to place

3. Wave Pulse A single disturbance that travels through a medium
A wave of short duration

4. Periodic Waves
If the disturbance repeats itself regularly then a periodic wave is generated.

5. Note
It is the disturbance that travels through the medium, not the medium itself.

6. Important
Regardless of the nature of the wave (sound, slinky, water, etc) or the medium through which it travels (air, solid, liquid, etc.) all waves exhibit wave-like behaviour (interference, diffraction, refraction, etc.) and can be described using the same terminology (amplitude, wavelength, speed, etc.) and the same mathematical equations
When I use sound waves to demonstrate and illustrate the Doppler effect for example, it is not because only sound waves exhibit this effect. In fact all waves exhibit this effect. It’s just easiest to demonstrate it with sound waves.

7. Here is the first exception …
Two basic types of waves:
If the disturbance is perpendicular to the direction of propagation of the wave it is called a transverse wave
If the disturbance is parallel to the direction of propagation of the wave it is called a longitudinal wave

9. crest
trough

10. Periodicity
Examples: pendulum, rocking chair, engine cycle, Earth’s orbit, etc.
When talking about anything that repeats itself at regular time intervals we can define …
The Period T: The time for one complete cycle
The frequency f: The number of complete cycles per unit time
What is the relationship between these two?

11. Example: Earth’s rotation
Period
Time Unit
Frequency 24
hours
1/24 1
days
1/7
weeks 7
1/365
years
365
Period and frequency are reciprocals

17. Assignment

18. Speed of a wave
The speed of a wave depends on the physical properties of the medium
Examples:
Waves travel faster through stiffer springs
Speed of waves in water depends on the depth
Sound waves travel faster in warmer air

19. Reflection
Fixed-end
Free end

20. Reflection and Transmission
Occurs at the boundary where the speed of wave changes

21. Interference What happens when two waves meet?
Wave interference occurs when two waves act simultaneously on the same particles of a medium. There are two types of interference: constructive and destructive.
destructive interference occurs when a crest meets a trough resulting in a lower amplitude
Constructive interference occurs when pulses build each other up, resulting in a larger amplitude (crest meets a crest or trough meets a trough)

22. The Nature of Sound
Longitudinal wave that is created by a vibrating object
Sound wave consists of traveling pulses of high pressure zones, or condensations, alternating with travelling pulses of low pressure zones, or rarefactions
These pressure fluctuations are normally very small

25. Sound requires a medium (gas, liquid, or solid)
Cannot exist in a vacuum

26. The frequency of a sound wave
A sound with a single frequency is called a pure tone
Human hearing
You can hear sounds from 20 Hz to Hz (20 kHz)
Me…up to about kHz
Infrasonic (<20 Hz) Ultrasonic (>20 kHz)
Rhinos use infrasonic (down to 5 Hz)
Bats use ultrasonic (up to 100 kHz)
The brain interprets the frequency detected by the ear in terms of pitch

27. High pitch corresponds to high frequency
Low pitch corresponds to low frequency

28. The Speed of Sound
Sound travels through gases, liquids, and solids, at considerably different speeds
At room temp in air its approx 343 m/s
Moves 4 times faster in water
Moves 17 times faster in solids
Speed increases with temp
Air (0°C) 331 m/s
Air (20°C) 343 m/s

29. Unless otherwise stated assume air at 20°C

31. Natural Frequency
Nearly all objects when hit, struck, plucked, or strummed will vibrate
They tend to vibrate at a particular frequency (or set of frequencies) called the natural frequency
Some tend to vibrate at a single frequency (pure tone) like a flute or a tuning fork while others vibrate at a set of frequencies

32. Example: Guitar String
The natural frequency of an object is determined by its physical properties
Example: Guitar String
Linear density
Tension
Length

34. Dropped Pencil
197 Hz
211 Hz
217 Hz
219 Hz
287 Hz
311 Hz
329 Hz
399 Hz
Flute
200 Hz
Tuba
200 Hz
400 Hz
600 Hz
800 Hz
1000 Hz

35. Forced Vibration
The tendency of one vibrating object to force another object into vibrational motion
A louder sound is always produced when an accompanying object of greater surface area is forced into vibration
Examples
Wooden body of a guitar
Sounding board in a piano

36. Resonance
Occurs when the frequency of forced vibration matches the natural frequency of the second object
The result is always a large vibration

37. Interference
Sound waves, like any waves, can be made to exhibit interference
When two or more sound waves from different sources are present at the same time, they interact with each other to produce a new wave.
The new wave is the sum of all the different waves.

38. Wave interaction is called interference.
If the compressions and the rarefactions of the two waves line up, they strengthen each other and create a wave with a higher intensity.
This type of interference is known as constructive.
When the compressions and rarefactions are out of phase, their interaction creates a wave with a dampened or lower intensity.
This is destructive interference.

41. Beats
A special case of interference occurs when two tones of slightly different frequencies are sounded together
A fluctuation in the loudness, called beats, is heard
The frequency of the beats is equal to the difference in frequencies
Example: If a 262 Hz and 266 Hz tuning forks are sounded together the beat frequency will be …
… 4 Hz !!!!

43. Diffraction
the bending of waves around obstacles and the spreading out of waves beyond openings.
Most pronounced when the wavelength is comparable to the size of the opening or obstacle

47. Doppler Effect
is the change in frequency of a wave for an observer moving relative to the source of the wave.
It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer.
The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession.

48. Doppler effect

49. What is light? Particle or wave?
Up until 1801 there were two competing theories –light as a wave and light as a particle
Both theories could explain the then known properties of light, namely reflection and refraction

50. Young’s Double Slit Experiment (1801)
Showed that light was a wave

51. James Clerk Maxwell
In 1865 he proposed the existence of electromagnetic waves produced by the vibration of electric charges
When he calculated the speed at which such waves should propagate it came out to a value very close to that of the speed of light
He concluded that light was a form of electromagnetic radiation

52. Electromagnetic Spectrum

53. But then …
Albert Einstein (1905) explained the photoelectric effect by theorizing that light is composed of particles whose energy depended on the frequency of the light
He called these particles photons

54. Wave-particle duality
Electromagnetic radiation propagates following linear wave equations, but can only be emitted or absorbed as discrete elements, thus acting as a wave and a particle simultaneously.
In 1924, Louis-Victor de Broglie formulated the de Broglie hypothesis, claiming that all matter, not just light, has a wave-like nature
This was confirmed 3 years later when electrons were observed to behave like waves and form interference patterns

55. The Speed of Light
Ole Romer first demonstrated in 1676 that light travelled at a finite speed (as opposed to instantaneously)
First accurate measurement was made in 1880 by Albert Michelson
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second

56. Refraction
Refraction is the change in direction of a wave due to a change in its speed.

59. Index of refraction

60. Snells Law

61. Laser Light
Light emitted by a common lamp has many different frequencies and it tends to spread out after a short distance becoming wider and wider and less intense with increased distance
It is said to be incoherent
A beam of light that has the same frequency and direction is said to be coherent
Coherent light is produced by a laser

62. Example
A laser emits a coherent light beam with a wavelength of 650 nm. What is the frequency of the wave?

64. n=4 fourth order
n=3 third order
n=2 second order
n=1 first order
n=o central bright
n=1 first order
n=2 second order
n=3 third order
n=4 fourth order

65. Links http://www.acoustics.salford.ac.uk/feschools/index.htm