Section 3 - Waves Flashcards
1
Q
Wave def
A
The oscillation of particles of fields
2
Q
Progressive wave def
A
Moving wave - carries energy from one place to another without transferring any material
3
Q
Evidence of how waves carry energy
A
- EM waves cause things to heat up
- X rays and gamma rays cause ionisation
- Sounds cause vibrations
- Wave power can be used to generate electricity
- Since waves carry energy away, the source loses energy
4
Q
Wave parts diagram
A
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5
Q
Wave Cycle
A
One complete vibration of the wave
6
Q
Wave Displacement
A
X in metres - how far from a point a wave has moved from its undisturbed position
7
Q
Wave Amplitude
A
A in metres, the maximum magnitude of displacement
8
Q
Wave Wavelength
A
In metres - the length of one whole wave cycle, from crest to crest or trough to trough
9
Q
Wave Period
A
T in seconds, the time taken for a whole cycle to complete or to pass given point
10
Q
Wave Frequency
A
f in hertz, the number of cycles per second
11
Q
Wave phase
A
A measurement of the position of a certain point along the wave cycle
12
Q
Wave phase difference
A
The amount one wave lags behind another
13
Q
Phase or Phase difference are measured as [2]
A
- Angles
- Fractions of a cycle
14
Q
Wave reflection def
A
The wave is bounced back when it hits a boundary
15
Q
Wave refraction def
A
The wave changes direction as it enters a different medium
16
Q
Frequency formula
A
f = 1/T
Where T = time period
17
Q
Wave speed formulae
A
Wave speed = Distance travelled / Time taken
c = d/t
18
Q
Speed, Wavelength, Ferquency
A
C = wavelength*freq
19
Q
Speed of light in a vacuum
A
3*10^8
20
Q
Transverse Waves def
A
Waves that oscillate at right angles to the direction of energy transfer
21
Q
Examples of transverse waves [3]
A
- All EM waves
- Ripples
- Waves on a string
22
Q
How can waves be drawn [2]
A
Displacement against distance
Displacement against time
23
Q
Longitudinal waves def
A
A wave in which the direction of oscillation in parallel to the direction of energy transfer
24
Q
A longitudinal wave consists of ___&___
A
Compressions and rarefactions
25
Examples of longitudinal waves
Sound waves, shock waves
26
Polarised Wave def
A wave that only oscillates in one direction
27
Polarisation can only happen for ______
Transverse Waves
28
How do polarising filters work [4]
- Light waves are a mixture of different directions of oscillation
- Waves can be polarised when passed through a polarising filter
- This causes them to oscillate in one direction only
- Two polarising filters at right angles will cause no light to pass through
29
How can light be partially polarised
When light is reflected off some surfaces, it can become partially polarised
30
How do Polaroid sunglasses work
If you view reflected partially polarised light through a polarising filter at the correct angle, you can block out unwanted glare
31
Polarisation of TV and radio signals
- TV and radio signals are polarised
- The rods of the transmitting and receiving Ariel must be aligned to receive a strong signal
32
The principle of superposition
When two or more waves cross, the resultant displacement equals the vector sum of the individual displacements
33
Interference can be ___ or ____
Constructive or destructive
34
Constructive interference
Two crests result in a bigger crest
35
Destructive interference
A crest and a trough of equal magnitude result in nothing since the two displacements cancel each other out
36
If two points are in phase, they interfere _____
Constructively
37
When are two points exactly in phase
When they are both at the same point in the wave cycle, have the same displacement and velocity
Phase difference of a multiple of 360 degrees
38
When are points exactly out of phase
When they have a phase difference of an odd multiple of 180 degrees
39
To get an interference pattern, the two sources ____ __ ________
must be coherent
40
What makes two sources coherent
Two sources are coherent if they have the same wavelength and frequency, thus a fixed phase difference between them
41
Constructive or destructive interference depends on __________
The path difference
42
Constructive interference occurs when
Path difference = n*wavelength
43
Destructive interference occurs when
Path difference = (n+0.5)Wavelengths
44
Stationary wave def
A stationary wave is the superposition of two progressive waves with the same frequency and wavelength, moving in opposite directions
45
Key difference between progressive waves and stationary waves
Progressive waves transfer energy, stationary waves do not
46
demonstrating stationary waves with a string
Set up a driving oscillator at one end of a stretched string with the other end fixed
47
Wave Node Def
Where the amplitude of the oscillation is zero
48
Wave antinode def
Points of maximum amplitude
49
Resonant frequency
At resonant frequencies, an exact number of half wavelength fit on the string
50
First harmonic
1/2 wavelengths
51
Second Harmonic
1 wavelength
52
Third Harmonic
3/2 wavelengths
53
Demonstrating stationary waves with microwaves
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54
Demonstrating stationary waves in a column of air
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55
Investigating factors affecting the resonant frequencies of a string
56
Factors affecting resonant frequencies on a string
Length
Weight
Tension
57
Frequency of fist harmonic
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58
Diffraction def
The way that waves spread out as they come through a narrow gap or go round obstacles
59
What does the amount of diffraction depend on
The wavelength of the wave compared to the size of the gap
60
Diffraction if the gap is a lot bigger than the wavelength
Diffraction is unnoticeable
61
Diffraction - If the gap is several wavelengths wide
Noticeable diffraction
62
Diffraction - the gap is the same size as the wavelength
Most diffraction
63
Diffraction - Gap is smaller than the wavelength
Waves are mostly reflected back
64
Conditions to observe a clear diffraction pattern
A monochromatic, coherent light source
65
Demonstrating light diffraction patterns with a laser
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66
How can light diffraction be demonstrated using a laser
Passing a wave through a narrow slit and projecting on a slit.
The wavelength is about the same size as the appeture
67
Observation for light diffraction using a laser [2]
A central bight fringe (central maximum)
Dark and bright fringes alternating on either side
68
How does the diffraction of white light create a spectra of colors [3]
- White light is a mixture of wavelengths
- When passed through the slit, all wavelengths are diffracted by a different amount
- This produces a spectra of colors
69
Light intensity [2]
The number of photons
Power per unit area
70
Light intensity of the central maximum
Highest Light intensity at the centre of the central maximum
71
Effect of increasing slit width on the width of the central maximum, light intensity [3]
Decreases amount of diffraction
Narrower central maximum
Higher light intensity
72
Effect of increasing wavelength on the width of the central maximum, light intensity [3]
Increases amount of diffraction
Wider Central Maximum
Lower light intensity
73
Demonstrating two source interference in water and sound
Two coherent sources with the same wavelength and frequency
74
Conditions for Youngs Double slit experiment
Two coherent monochromatic sources, the slit size is about the same as the wavelength
75
Youngs double slit setup
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76
Laser Safety [5]
1) Never shine the laser towards a person
2) Wear laser safety goggles
3) Avoid shining the laser beam at a reflective surface
4) Have a warning sign on display
5) Turn the laser off when it is not needed
77
Microwaves two source interference setup
IMAGE
78
Youngs double slit fringe spacing formula
Fringe Spacing = (Wavelength*Distance)/slit spacing
79
How is the Youngs Experiment evidence for the Wave nature of EM radiation [2]
- Diffraction and interference are both uniquely wave properties
- The experiment shows that light can diffract and interfere
80
Interference patterns get ______ when you diffract through more slits
Sharper, the multiple beams reinforce the pattern
81
Why are sharper beams more useful
More accurate measurements
82
Observations for a monochromatic diffraction grating
1) All the Maxima are sharp lines
2) There is a line of maximum brightness at the centre called the zero order line
3) The following Paris are called the first order lines and so on
83
Diffraction grating formula
slit spacing* sin () = n wavelength
84
Diagram of diffraction gratings for monochromatic light
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85
Deriving the equation for diffraction gratings
- triangle, d sin a = wavelength
86
Diffraction gratings, bigger wavelength
Sin () is bigger
so () is bigger
The pattern is more spread out
87
Diffraction gratings, bigger slit spacing
Sin () is smaller
() is smaller
The pattern is less spread out
88
Diffraction gratings for values of sin() >1
Do not exist since sin() can not be greater than 1
89
What happens when diffraction gratings are used for non-monochromatic light
- White light is made of a mix of wavelengths
- Different wavelengths diffract different amounts
- Each order in the pattern becomes a spectrum
- Zero order stays white
90
Image, diffraction of white light through a grating
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91
Uses of diffraction gratings [4]
- More accurate
- Called X ray crystallography
- Used to measure the spacing between atoms
- Used to discover DNA
92
Refractive index def
A measure of how much light slows down in a material / how much light will defreact
93
Why diffraction happens
Light slows down in materials because it interacts with the particles in it. The more dense the material is, the more light slows down. The slowing down causes light to bend at a boundary
94
Formaula for absolute refractive index
n = Speed of light in a vacuum / speed of light in medium
95
Formula for relative refractive index
1n2 = c1 / c2
96
relative refractive index in terms of absolute refractive index
1n2 = n2 / n1
97
Absolute refractive index vs relative refractive index
the absolute refractive index is the property of a single material only while the relative refractive index is the property of the interface between two materials
98
Refractive index of air / vaccum
1
99
When light enters a denser medium
Refraction towards the normal
100
When light enters a less dense medium
Refraction away from the normal
101
Snells Law
n1 * sin()1 = n2 * sin()2
102
Critical Angle def
The maximum angle unto which refraction occurs. At () greater than the critical angle, all the light is reflected back into the material, total internal reflection
103
Critical angle formula
1n2 = sin ()c
104
How do optical fibres work [3]
1) Optical fibres are made of a material with a high refractive index which is surrounded by a material with a low refractive index
2) This means that light is totally internally reflected
3) light bounces from side to side at angles greater than the CIR, moving along the firbre
105
Why is the cladding needed on an optical fibre
Protects fibres from scratches which would cause light to escape
Allows TIR to happen
106
Absorption in optical fibres
Causes loss in amplitude
Energy is absorbed by the material
107
Modal Dispersion in optical fibres
Light rays enter at different angles and take different paths, some take a longer path
108
Preventing modal dispersion in optical fibres
Single mode fibre that only lets light take one path, preventing modal dispersion
109
Material dispersion in optical fibres
Light consists of different wavelengths so some wavelengths reach the end of the fibre faster than others
110
Preventing material dispersion in optical firbres
Using monochromatic light
111
Result of dispersion in optical fibres
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pulse broadens at each end
