Waves Flashcards
1
Q
What is a wave?
A
A regular disturbance that carries energy from one place to another.
2
Q
What does a wave transport?
A
Energy, not matter
3
Q
When a wave is present in a medium, what happens to the individual particles?
A
They are temporarily displaced from their rest position; there is always a force acting upon the particles that restores them to their original position.
4
Q
What are the two ways of showing wave motion in a graph?
A
- displacement-time
- displacement-distance
5
Q
What is displacement?
A
Instantaneous distance from the equilibrium level
6
Q
What is amplitude?
A
The maximum displacement from the equilibrium position
7
Q
What is wavelength?
A
The distance between any two points on adjacent cycles which are vibrating in phase.
8
Q
What is the meaning of ‘in phase’?
A
At the same point in the cycle
9
Q
What is time period?
A
The time taken for one complete oscillation
10
Q
What is frequency?
A
The number of oscillations in one second
11
Q
What do the symbols stand for? T = 1/f
A
T = time period, f = frequency
12
Q
What do the symbols stand for? c = fλ
A
c = wave speed, f = frequency, λ = wavelength
13
Q
Derivation of the wave equation?
A
Speed = distance / time
Speed = wavelength / period
Speed = wavelength / (1/frequency)
Speed = wavelength x frequency
14
Q
What is phase difference?
A
The difference between two waves having the same frequency and referenced to the same point in time
15
Q
What is phase difference expressed in?
A
Degrees, radians or fractions of a cycle
16
Q
Would two oscillators with the same frequency and different phases have a phase difference?
A
Yes - they would be out of phase with each other
17
Q
What is the range of values for phase difference?
A
- Degrees - 0 to 360
- Radians - 0 to 2π
18
Q
What is antiphase?
A
When the phase difference is 180 degrees (π radians)
19
Q
What is the equation for phase difference?
A
x/λ x 360 (degrees)
20
Q
What are transverse waves?
A
When the displacement is at right angles to the direction of energy propagation
21
Q
What are longitudinal waves?
A
When the displacement is parallel to the direction of energy propagation
22
Q
What type of wave is light?
A
Transverse, electromagnetic
23
Q
What type of wave is sound?
A
Longitudinal, mechanical
24
Q
What are mechanical waves?
A
Waves that travel by vibrating particles in a medium
25
Can mechanical waves travel in a vacuum?
No
26
What are electromagnetic waves?
Waves that can travel through a vacuum
27
What is speed of light in a vacuum?
3 x 10⁸ m/s
28
What happens when an electromagnetic wave hits a surface?
The wave can be reflected, transmitted or absorbed
29
What happens to an object when it absorbs an electromagnetic wave?
Its temperature increases
30
In which planes can the displacements of oscillations in transverse waves be?
In all planes
31
What are plane-polarised waves?
Have the oscillations in one plane only
32
How is light polarised?
- by absorbing all planes of oscillation except one
- by reflection
33
What is Polaroid plastic formed from?
Many tiny crystals, all lined up
34
Which planes of oscillations does Polaroid plastic absorb?
All of them except the vertical one
35
When light is polarised by reflection, which way is the plane of polarisation?
Horizontal
36
What happens if you wear sunglasses made from Polaroid plastic and look at water?
Reflection off the water surface is absorbed because its plane of polarisation is perpendicular to that of the Polaroid
37
What happens if two pieces of Polaroid are 'crossed so that their transmission planes are at right angles?
No light will get through
38
How can transverse and longitudinal waves be distinguished?
Transverse waves can be polarised; longitudinal cannot
39
What are electromagnetic waves a combination of?
Electric and magnetic field waves produced by moving charges
40
What is polarisation used in?
- sunglasses
- alignment of aerials for transmission and reception
41
When are superposed?
When two waves of the same type are in the same place at the same time
42
How is the resultant displacement at any point found when two waves are superposed?
By adding displacements of each separate wave
43
What is interference?
The adding together of waves
44
What is the principle of superposition?
At a point where two or more waves meet, the instantaneous displacement (amplitude) is the vector sum of the individual displacements due to each wave at that point
45
When will interference be constructive?
When waves are in phase and the same frequency
46
What must the path difference be for constructive interference?
nλ (where n is a whole number of wavelengths)
47
When will interference be destructive?
When waves are in antiphase and have the same frequency
48
What must the path difference be for destructive interference?
(1 + n/2)λ i.e. an odd number of wavelengths
49
What does it mean when waves are phase linked?
The waves have a constant phase difference
50
When are superposed waves easier to 'see'?
- the waves are of similar amplitude (increased constant between maxima and minima)
- the waves have similar frequencies (otherwise the interference patterns change so fast that they are difficult to detect)
- the waves have a constant phase difference
51
What are some examples of coherent sources?
- Light produced by a laser
- Sound from two loudspeakers connected in parallel
- Light emerging from two apertures illuminated by the same source
52
What are coherent sources?
Sources that have synchronised phase changes, as well as the same frequency and wavelength
53
What are nodes?
On stationary waves, points that are always at equilibrium and zero oscillation
54
What are antinodes?
On stationary waves, points of maximum oscillation
55
On a stationary wave, what is the distance from one node to the next?
1/2 λ
56
How are stationary waves formed on a string?
- Vibrator moves up and down, sending travelling wave down the cord
- Wave is reflected at the end, so the two travelling waves overlap and interfere
- Has antinodes and nodes; distance between nodes = 1/2λ
57
Comparison of the frequencies of particles in stationary and travelling waves?
Stationary - all particles (except nodes) have the same frequency
Travelling - all particles have the same frequency
58
Comparison of the amplitudes of particles in stationary and travelling waves?
Stationary - varies from 0 (nodes) to maximum (antinodes)
Travelling - same for all particles
59
When does resonance occur?
When the frequency driving the system matches the natural frequency of the system
60
Comparison of the phase difference between two particles in stationary and travelling waves?
In stationary waves, points between two adjacent nodes or separated by an even number of nodes are in phase, whereas points separated by an odd number of nodes are in antiphase. (mπ where m is the number of nodes between the two particles).
In travelling waves phase difference is (2πx/λ where x is the distance apart)
61
Comparison of the energy of particles in stationary and travelling waves?
Stationary - energy stored and not transferred
Progressive - energy transferred
62
What is a stationary wave?
Where energy is stored rather than transmitted - formed when two coherent waves travelling in opposite directions interfere to produce nodes and antinodes
63
What can increase the pitch of a note on a guitar string?
- Increase the tension
- Decrease the length of the string
- Decrease the thickness of the string
64
What does the first harmonic depend on?
Tension T, length l, and its mass per unit length
65
What do the symbols stand for? f = (1/2L) x √(T/μ)
f = frequency, l = length, T = tension, μ = mass per unit length
66
What happens when the air at one end of the tube/pipe is caused to vibrate?
A longitudinal wave travels down the tube and is reflected at the opposite end, forming a stationary wave.
67
Where are the anti-nodes in an open pipe?
At both ends
68
Why are waves reflected at the ends of open pipes?
Air acts as a barrier outside
69
At resonant frequencies in a closed pipe, where are the nodes and anti-nodes?
Node at closed end, anti-node at open end
70
Describe the amplitude of the particles in a closed pipe.
Amplitude decreases gradually from the maximum at the open end to zero at the closed end
71
Which harmonics can be obtained in an open pipe?
All of them
72
Which harmonics can be obtained in a closed pipe?
Odd harmonics
73
Why are standing waves only produced at certain frequencies?
There needs to be a whole number of stationary wave loops fitting into the length of the string
74
What does the double slit interference pattern consist of?
Equidistant parallel fringes alternating between: maxima (constructive interference) and minima (destructive interference).
75
What happens when waves are travelling in the same direction and overlap?
They interfere
76
What does it mean if two sources are coherent?
They emit identical waves which start in phase
77
How can light that is in phase be produced for the double slit experiment?
- use two coherent sources
- use a single source with double slits
78
What does it mean if two sources are coherent?
They have the same frequency, wavelength and synchronised phase changes
79
What do the symbols stand for?
w = λD / s
w = fringe spacing, λ = wavelength, D = slit to screen distance, s = spacing between slits
80
For small angles, what does sinθ equal?
θ
81
If all types of wave interfere, why can't we see interference patterns?
To obtain a clear interference pattern, it requires two coherent waves of monochromatic light. Light is usually emitted in bursts of waves, after which is a random phase change.
82
How is light usually emitted?
In bursts of waves, each burst lasting 10⁻⁹ s, after which there's a random phase change.
83
What is a monochromatic source?
A source of a single wavelength
84
Is there interference when two separate light sources are used?
Never
85
What is fringe separation?
The distance between neighbouring bright fringes
86
What happens to the double slit interference pattern if green light is used instead of red?
Wavelength is decreased so distance between adjacent fringes decreases
87
What happens to the double slit interference pattern if white light is used?
The central fringe is white with red edges. Other fringes will be spectra with the blue end towards the middle of the overlap area.
88
What happens to the double slit interference pattern if the screen is moved further awat?
D increases so the distance between adjacent fringes increases
89
What happens to the double slit interference pattern if the phase difference between two sources is changed to 180 degrees?
The maxima will become minima and vice versa
90
What happens to the double slit interference pattern if both slits are made narrower?
Wider interference so there are more dots, but fainter as there is less light through (x increases)
91
What happens to the double slit interference pattern if one slit is narrower than the other?
The waves don't fully cancel out
92
How can you increase x in the Young's slit experiment?
Increasing D - measurement is easier and more accurate but fringe intensity decreases
Decreasing a - practical limit to this
Increasing wavelength
93
Can mechanical waves interfere?
Yes
94
What is diffraction?
When a wave passes through a gap and spreads out
95
What happens to diffraction when the gap width decreases?
Diffraction increases
96
When is diffraction strongest?
When the gap width is similar to the wavelength of the wave
97
Why do waves passing through a single gap interfere?
Only one slit but more than one wave. The single slit can be though of as a large number of sources next to each other. Each 'source' produces a coherent wave, which overlap and interfere.
98
What happens, when light is shone on a diffraction grating, when the wavelength is increased?
Short λ (like blue light) has a narrow diffraction pattern
Long λ (like red light) has a broad diffraction pattern
99
What happens when white light is shone on the diffraction grating instead of monochromatic?
White light yields less clear patterns (as position of dark bands depends on λ)
Colours appear; only central band is white
100
What is the difference between a single and double slit pattern?
Single slit - central maximum is twice the width of the other fringes
Double slit pattern has equally spaced fringes
101
How many slits are on a diffraction grating?
1000s
102
Which method produces a better diffraction pattern?
Diffraction grating - as not much light gets through the doubles slits so are dim and unclear
103
What is a diffraction grating?
A set of slits for light waves to pass through
104
How do you calculate the number of slits per meter on a diffraction grating?
x = 1/d
105
What do the symbols stand for?
nλ = dsinθ
n = order of maxima, λ = wavelength, d = slit separation, θ = angle
106
In the diffraction grating equation, what is the significance of sinθ never being greater than one?
There is a limit to the number of spectra that can be obtained
107
What is the zero-order maximum?
The waves that produce the bright spot straight on. Paths are all the same length, so phase difference is zero
108
What is the equation for the maximum number of orders?
n = d/λ
109
Why is sinθ not present in the equation for the maximum number of orders?
It will give a maximum when sinθ = 1 so cancels out
110
When using the equation n = d/λ, which quantity must be a whole number?
n
111
Which is more accurate, the diffraction grating or the double slit method?
Diffraction grating
112
Why is the diffraction grating more accurate than the double slits?
Double slits - fringes formed are slight blurred so there are large errors
Diffraction grating - images are clear and measurements accurate. Final result is an average of several calculations
113
What can diffraction gratings be used for?
Analysis of spectra
114
What is an optical fibre?
A long, thin, cylindrical core of glass, encased in a cladding of glass of lower refractive index
115
What is refraction?
A change in the direction of light as it passes across a boundary between two transparent substances
116
What happens in terms of refraction, if light passes across a boundary at 90 degrees to a surface?
It doesn't refract
117
What is a refractive index?
A measure of the optical density of a material relative to air
118
What is the approximate refractive index of air?
1
119
What does the symbols stand for?
n=c/cs
n = refractive index, c = speed of light in a vacuum, cs = speed of light in the medium
120
What is the definition of Snell's law?
The ratio of the sines of the angles of incidence and refraction are constant when it passes between two given media
121
What do the symbols stand for?
n₁sinθ₁ = n₂sinθ₂
n = refractive index, θ = angle
122
What are the two conditions for total internal reflection?
- Light passes from a more optically dense medium to a less optically dense medium
- Angle of incidence > critical angle
123
What is total internal reflection?
When light passes from a more to less dense medium and the angle of incidence is greater than the critical angle, all light is reflected back to the less dense medium
124
What is the critical angle?
The angle of incidence to which the angle of refraction is 90 degrees
125
What is the angle of refraction when the angle of incidence is equal to the critical angle?
90°
126
What do the symbols stand for?
sinθc = n₁ / n₂
θc = critical angle, n₁ = the first material, n₂ = the material it enters
127
How does light travel along an optical fibre?
By total internal reflection, only escaping when it reaches the other end
128
What is an endoscope?
A medical instrument that uses optical fibres to look inside the body
129
What do endoscopes consist of?
A coherent bundle of fibres (lens system) and an incoherent bundle of fibres (light delivery system)
130
What happens if a fibre is bent too tightly?
Angle of incidence will be less than the critical angle and light will escape
131
What can endoscopes be used to look at?
Digestive, respiratory and female reproductive systems
132
What are the positives of endoscopes?
Can diagnose patients without an incision, often without anesthetic
133
In an optical fibre, when will total internal reflection occur?
As long as θ is larger than the critical angle
134
In medicine, what are the uses of optical fibres?
Endoscopes, lasers - burn tissue to heal wound
135
What is a coherent bundle of fibres?
Where the fibres stay in the same relative position along their length
136
What are some of the problems for optical fibres?
- Scratches can cause light to leak
- Two fibres touching can cause light to pass from one to the other
- Dispersion
137
How can scratches and cross talk be resolved when using optical fibres?
Using cladding
138
Does cladding have a lower or higher refractive index than the core?
Lower
139
How is light sent down an optical fibre?
In pulses or bursts
140
How can a pulse be distorted in an optical fibre?
Absorption - some energy is absorbed so the pulse has a lower amplitude
Dispersion - causes pulse broadening
141
What are the two types of dispersion?
Modal - light takes different paths through the core
Material - different wavelengths of light travel at different speeds
142
How does modal dispersion occur?
A pulse can take a variety of different paths through a fibre, meaning a single pulse can spread out over time
143
How can modal dispersion be decreased?
Use a narrow core
144
How does material dispersion occur?
Different wavelengths of light travel at different speeds through the core. A pulse will reach the end of the fibre at different times if there are multiple wavelengths, which is pulse broadening
145
How can material dispersion be reduced?
Using monochromatic light
146
What colour of light should be used in an optical fibre
Red - it travels faster
147
What is it called when, in a prism, white light is split into a spectrum of colours?
Dispersion
148
In a prism, what colour light is refracted more: red or blue?
Blue
149
Why is blue light refracted more than red in a prism?
Blue light travels more slowly in glass than red light
150
When is pulse distortion more of a problem?
When the pulses are very short and close together
151
When are single fibres used?
In communications
152
When are bundles of fibres used?
In endoscopes
153
What is pitch?
Pitch is a term used to describe how high or low a note seems to be. The pitch of a note depends on the frequency
