Waves Flashcards
(184 cards)
1
Q
What is a progressive wave?
A
Vibrations through the air that carries energy from one place to another without transferring any material
2
Q
How is a wave produced?
A
Caused by something making particles or fields oscillate at a source
3
Q
What happens to the energy at the source of a wave?
A
It decreases as the wave transfers energy away from the source
4
Q
What is reflection?
A
When a wave bounces back when it hits a boundary
5
Q
What is refraction?
A
When a wave changes direction as it enters a different medium
6
Q
Why does a wave change direction (refracts) when it enters a different medium?
A
Because the wave speeds up or slows down
7
Q
What is diffraction?
A
When a wave spreads out as it passes through a gap or round an obstacle
8
Q
What is the displacement of a wave?
A
How far a point on the wave has moved from it’s undisturbed position
9
Q
What is the SI units of the displacement of a wave?
A
Metres
10
Q
What is the amplitude of a wave?
A
The maximum displacement of a wave (e.g. distance from undisturbed position to peak or trough)
11
Q
What is the SI units of the amplitude of a wave?
A
Metres
12
Q
What is the wavelength of a wave?
A
The length of one whole wave cycle (e.g. distance from peak to peak or trough to trough)
13
Q
What is the period of a wave?
A
The time taken for one whole wave cycle
14
Q
What is the SI units of the wavelength of a wave?
A
Metres
15
Q
What is the SI units of the period of a wave?
A
Seconds
16
Q
What is the frequency of a wave?
A
The number of whole wave cycles per second passing a given point
17
Q
What is the SI units of the frequency of a wave?
A
Hertz
18
Q
What is the phase of a wave?
A
A measurement of the position of a certain point along the wave cycle
19
Q
What is the phase difference of a wave?
A
The amount by which one wave is behind another
20
Q
What is the phase and phase difference of a wave measured in?
A
Degrees or radians as an angle or in fractions of a cycle
21
Q
How do you work out the phase difference of 2 different waves?
A
Work out the difference between 2 identical points on each wave (e.g. the peak of both waves)
22
Q
What equation links frequency and time period?
A
F = 1 / T
23
Q
What is the wave speed?
A
The speed that a wave travels at
24
Q
What is the equation for wave speed?
A
Wave speed = Frequency x Wavelength
25
What speed do all electromagnetic waves travel at through a vacuum?
3x10^8 ms^-1
26
What is a transverse wave?
A wave where the oscillations are at right angle to the direction of energy propagation
27
Are EM waves transverse or longitudinal?
Transverse
28
How do transverse waves travel?
As vibrations through magnetic and electric fields
29
What are 3 examples (except EM waves) of a transverse wave?
Waves on a string
Waves on water
Seismic S-waves
30
What are the 2 types of graphs you can draw to represent transverse waves?
Displacement-Distance graph
| Displacement-Time graph
31
What is a longitudinal wave?
A wave where the oscillations are parallel to the direction of energy propagation
32
What is an example of a longitudinal wave?
Sound waves
33
What does a sound wave consist of?
Alternate areas of compression and rarefaction of the medium it's travelling through
34
Why can't sound waves travel in a vacuum?
Because it has areas of compression and rarefaction
35
Is a Seismic P-wave transverse or longitudinal?
Longitudinal
36
How do you find the wavelength of a longitudinal wave?
It's the distance between compressions
37
What is a polarised wave?
A wave that only oscillates in one direction (as normal waves oscillate in all directions on the plane perpendicular to the direction of motion)
38
What is a polarising filter?
A filter that only transmits vibrations of a wave in one direction (the plane of transmission)
39
What is the plane of transmission?
The only direction of vibrations that a polarising filter transmits
40
What will you see if you hold up 2 polarising filters at right angle to each other?
Nothing as no light will get through
41
What does the transmission axis of a polarising filter show?
The only direction of vibrations which can pass through the filter
42
What is the only type of wave that polarisation can occur?
Transverse waves
43
What are 2 uses of polarisation in the real world?
Glare reduction
| Improving TV and radio signals
44
When does superposition occur?
When 2 waves pass through each other
45
What is superposition?
The combination of displacements experienced when 2 2 waves pass through each other
46
What can the superposition of 2 waves result in?
Interference
47
What are the 2 types of interference?
Constructive
| Destructive
48
What is constructive interference?
When 2 waves interfere to make a wave with a larger displacement
49
What is destructive interference?
When 2 waves interfere to make a wave with a reduced displacement
50
What is total destructive interference?
When 2 waves meet with equal and opposite displacements meet and cancel each other out completely
51
Describe what is meant by 2 waves in phase?
Where 2 points on a wave are in phase if they're both at the same point in a wave cycle
52
What does a phase difference of 180° (π radians) mean?
The 2 waves are out of phase
53
What is a stationary wave?
A wave that is the superposition of 2 progressive waves travelling in opposite directions with the same amplitude, frequency and wave speed
54
How much energy is transmitted by a stationary wave?
None, because the 2 waves are carrying equal amounts of energy but in opposite directions
55
What happens at the resonant frequency?
The wave forms a harmonic
56
What is a node for a stationary wave?
A point at which the amplitude of the vibrations is 0
57
What is an antinode for a stationary wave?
A point at which the amplitude of the vibration is at maximum
58
Describe what happens to a length of string at a resonant frequency?
An exact number of wavelengths fit onto the length of string forming a harmonic
59
What type of interference happens at a node?
Total destructive interference
60
What type of interference happens at an antinode?
Constructive interference
61
Describe the first harmonic in terms of number of wavelengths and the number of 'loops'?
One half wavelength fits on the string
| 1 loop with a node at each end
62
Describe the second harmonic in terms of number of wavelengths and the number of 'loops'?
Two half wavelengths fit on the string (1 wavelength)
| 2 loops with a node in the middle and at each end
63
Describe the third harmonic in terms of number of wavelengths and the number of 'loops'?
3 half wavelengths fit on the string
| 3 loops with 4 nodes in total
64
How many wavelengths will fit on a string at the A harmonic?
A/2 wavelengths
65
What is the equation for mass per unit length of the string, μ?
μ = Mass / Length
66
What is the SI unit for mass per unit length?
Kg m^-1
67
What is the equation for the tension of a string?
T = Mass x Gravity
68
What is the SI unit of tension?
Newtons
69
What is the equation for calculating the frequency of the first harmonic?
f = (1 / 2 x length) x (√(tension / mass per unit length))
70
What is the affect of a longer string length on the resonant frequency?
It means a lower resonant frequency, because the half-wavelength is longer
71
What is the affect of a higher mass per unit length of string on the resonant frequency?
It means a lower resonant frequency, because waves travel more slowly down the string
72
What is the affect of a greater tension on the string for the resonant frequency?
A higher resonant frequency, because waves travel faster on a more taught string
73
What is it called when waves spread out as they come through a narrow gap or go around narrow obstacles?
Diffraction
74
How much diffraction occurs when the gap is a lot bigger than the wavelength?
Diffraction is un-noticeable
75
How much diffraction occurs when waves pass through a gap that is several wavelength wide?
A little bit of diffraction occurs
76
When does the most diffraction occur?
When the gap is the same size as the wavelength of the ray
77
What happens if the gap between 2 slits is smaller than the wavelength?
Most of the waves are reflected back
78
Describe diffraction around an object
When a wave meets an object, you get diffraction around the edges as behind the object is a shadow, where the object is blocking the wave
79
What is a monochromatic light source?
A light of a single wavelength (and frequency) and so a single colour
80
Why do you have to use monochromatic when you want to observe a clear pattern of diffraction on a screen?
Because light that isn't monochromatic has different wavelengths so will diffract at different amounts, producing an unclear pattern
81
What is meant by 'coherent' light?
2 waves are coherent if they have the same wavelength and frequency and a fixed phase difference between them
82
Why is laser light used for seeing diffraction?
Because it's a coherent, monochromatic source of light
83
Which has a higher frequency of light, red or blue light?
Blue light
84
Which has the longer wavelength, red or blue light?
Red light
85
Describe the pattern on the screen produced by the single-slit experiment
A bright central fringe (twice as wide as other fringes) with alternating dark and bright fringes on either side
86
What causes the fringe pattern for the single-slit experiment?
Interference
87
For the single-slit experiment, what happens at the bright fringes?
Constructive interference, where waves from across the width of the slit arrive at the screen in phase
88
For the single-slit experiment, what happens at the dark fringes?
Total destructive interference, where waves from across the width of the slit arrive at the screen completely out of phase
89
What happens when you shine white light through a single slit?
It diffracts to from a spectra of colours for each of the bright fringes
90
Why do you get a spectra of colours when you shine white light through a single slit?
Because all the different wavelengths of light are diffracted by different amounts
91
What is intensity?
The power per unit area
92
For monochromatic light, what can be said about the photons?
They have the same energy?
93
In terms of number of photons, what does an increase in intensity mean?
An increase in the number of photons per second
94
How does the width of the central maximum change with an increasing slit width, for the single-slit experiment?
The central maximum is narrower, because a wider slit width decreases the amount of diffraction
95
How does the intensity of the central maximum change with an increasing slit width, for the single-slit experiment?
Intensity of the central maximum is higher
96
How does the width of the central maximum change with an increasing wavelength, for the single-slit experiment?
Increasing the wavelength increases the amount of diffraction, so this means that the central maximum is wider
97
How does the intensity of the central maximum change with an increasing wavelength, for the single-slit experiment?
Intensity of the central maximum is lower
98
What is two-source interference?
When waves from 2 sources interfere to produce a pattern
99
In order to get a clear two-source interference pattern, what do the sources need to be?
Monochromatic and coherent
100
What is meant by coherent?
2 waves are coherent if they have the same frequency, wavelength and a fixed phase difference
101
Describe the intensity of the beam from a coherent light source
A very high intensity beam, because the troughs and peaks of the waves line up causing constructive interference
102
Is visible light coherent or non-coherent?
Non-coherent
103
Is laser light coherent or non-coherent?
Coherent
104
Why do non-coherent sources not produce a clear interference pattern?
Because the different wavelengths and frequencies means waves are diffracted differently, so interference is a jumble
105
What is a maxima?
A point in an interference pattern where there is constructive interference
106
What does the path difference have to be to get constructive interference?
A whole number of wavelengths
107
What is a minima?
A point in an interference pattern where there is total destructive interference
108
What does the path difference have to be to get total destructive interference?
A whole number of wavelengths + 1/2 wavelength (e.g. 0.5, 1.5, 2.5, 3.5)
109
Who was the first person to do the Double-Slit Experiment?
Thomas Young
110
For the double-slit experiment, what does the width of the slits have to be roughly equal to?
The wavelength of the laser light
111
What does the diffraction pattern look like for the double-slit experiment?
A pattern of light and dark fringes
112
For the double-slit experiment, if you were to use a white source of light rather than a coherent laser light source, what would be the 3 differences for the diffraction pattern?
- The maxima would be wider and less intense
| - The pattern would contain a mix of colours with a central white fringe
113
What are the health risks of using laser light?
Can cause permanent eye damage, as the light is incredibly intense so would damage your retina
114
What are 3 safety precautions when using a laser light source?
- Never shine the laser towards peoples eyes
- Have a warning sign on display
- Turn laser off when not needed
115
What is the equation for the double-slit experiment?
W = (λD) / S
116
For the equation for the double-slit experiment, what is W equal to?
Fringe spacing, m
117
For the equation for the double-slit experiment, what is λ equal to?
Wavelength, m
118
For the equation for the double-slit experiment, what is D equal to?
Distance between slits and screen, m
119
For the equation for the double-slit experiment, what is S equal to?
Slit spacing, m
120
For the double-slit experiment, what is the path difference of the 2 waves forming a maxima?
λ
121
For the double-slit experiment, how does increasing the distance from the slits to screen affect the fringe?
Fringe spacing increases
122
For the double-slit experiment, how does increasing the slit separation affect the fringe spacing?
Fringe spacing decreases
123
For the double-slit experiment, how does changing the light source for one with longer wavelength affect the fringe spacing?
Fringe spacing increases
124
What provides the evidence for the wave-like nature of light?
Double-slit experiment
125
How does the double-slit experiment act as evidence for the wave-like nature of light?
It shows that light can diffract through a gap and interfere
126
What is the range of wavelengths of visible light in metres?
4.00×10^−7 to 7.00×10^−7 m
127
What is a diffraction grating?
A slide that contains lots of equally spaced slits very close together
128
Describe the diffraction pattern when you shine a source of monochromatic light at right angle through a diffraction grating
A pattern of equally spaced bright dots
129
Why is the interference pattern from a diffraction grating very sharp and intense?
Because there are so many beams reinforcing the pattern (because there's so many slits in the diffraction grating)
130
What is the zero order line?
The line of maximum brightness at the centre of a diffraction grating interference pattern
131
What is the equation for the angle between incident ray and the nth order maximum?
d sinθ = nλ
132
For equation to find the angle between incident ray and the nth order maximum, what does d equal?
Distance between slits, m
133
For equation to find the angle between incident ray and the nth order maximum, what does θ equal?
Angle to the normal made by the maximum, radians or degrees
134
For equation to find the angle between incident ray and the nth order maximum, what does n equal?
Order of maximum
135
For equation to find the angle between incident ray and the nth order maximum, what does λ equal?
Wavelength of light source, m
136
For the diffraction grating interference pattern, what affect on θ does increasing the wavelength?
θ will be bigger, so pattern spreads out more
137
For the diffraction grating interference pattern, what affect on θ does increasing the distance between slits?
θ is smaller, the pattern will be less spread out
138
How do you work out the maximum order ray?
n = d/λ (This is because you can't have a value of sinθ that's greater than 1)
139
Describe the diffraction grating interference pattern if you shine white light through the diffraction grating
Each order will become a spectra, with red on the outside and violet on the inside. The zero order stays white because the wavelengths just pass straight through without diffracting
140
What is a line absorption spectra?
A light spectrum with dark lines that correspond to different wavelengths of light that have been absorbed
141
What do you get if you split up light from a star using a diffraction grating?
A line absorption spectra
142
Where does light travel the quickest?
In a vacuum
143
Why does light travel slower in other materials than a vacuum?
Because it interacts with the particles in the material
144
What is optical density
The property of a material that defines how fast light goes through it
145
The more optically dense a material is...?
The more optically dense a material is the slower light travels through it
146
The higher a materials refractive index...?
The higher a materials refractive index, the more optically dense the material is
147
What is the refractive index of a material?
The ratio between the speed of light in a vacuum and the speed of light in the material
148
What is the equation for the refractive index of the material?
n = C / Cs
149
For the equation to find the refractive index of the material, what does C equal?
Speed of light in a vacuum = 3.00x10^8 ms^-1
150
For the equation to find the refractive index of the material, what does Cs equal?
Speed of light in the material, ms^-1
151
What is the refractive index of air?
1
152
What are the units of refractive index?
No units, as it's just a ratio
153
What is the relative refractive index?
The ratio of the speed of light in material 1 to the speed of light in material 2
154
What are the equations for the relative refractive index?
1n2 = C1 / C2 or 1n2 = n2 / n1
155
What is the angle of incidence?
The angle that incoming light makes to the normal
156
What is the angle of refraction?
The angle that the refracted ray makes with the normal
157
What is the equation for Snell's Law of Refraction?
n1 sinθ1 = n2 sinθ2
158
Which way does light bend if it passes from a less optically dense material to a more optically dense material?
Bends towards the normal
159
Which way does light bend if it passes from a more optically dense material to a less optically dense material?
Bends away from the normal
160
What is the critical angle?
The angle of incidence at which the angle of refraction is 90°
161
What does the refracted ray look like on a diagram if the angle of incidence = the critical angle?
The light is refracted along the boundary
162
What are the conditions needed for there to be a critical angle?
Light needs to be passing from a more optically dense material to a less optically dense material
163
What is the formula for the critical angle?
sinθ = n2 / n1 (where n1 is greater than n2)
164
What happens if the angle of incidence is greater than the critical angle?
Total internal reflection, because refraction can't happen
165
What is an optical fibre?
A very thin flexible tube of glass that can carry light signals over long distances and around corners using TIR
166
Describe what a step-index optical fibre looks like
A high refractive index core surrounded by cladding with a lower refractive index
167
What are the 2 reasons for optical fibres having cladding?
So total internal reflection can occur
| Protects the fibres from scratches which could let light escape
168
How do optical fibres work?
Light is shone in at one end of fibre. The fibre is so narrow that the angle of incidence is always greater than the critical angle, so total internal reflection happens from one end to the other
169
What are 3 advantages of using optical fibres?
There is no electrical interference
Fibre-optic cables are cheaper to produce
The signal can carry more information, because light has a higher frequency
170
What are the 2 types of signal degradation?
Absorption
| Dispersion
171
What is absorption for optical cables?
Where some of the signal's energy is absorbed by the material the fibre is made up of
172
Why is absorption a problem when using optical fibre cables?
The energy loss results in the signals amplitude decreasing over distance
173
What are the 2 types of dispersion?
Modal dispersion
| Material dispersion
174
What does dispersion cause?
Pulse broadening
175
What is pulse broadening?
The received signal is wider than the initial signal
176
What does pules broadening cause?
Information to be lost, because broadened pulses can overlap each other
177
What is modal dispersion caused by?
Light rays entering the optical fibres at different angles
178
Why is modal dispersion a problem?
Because signals arrive at different times, because the rays taking a straight path are quicker than rays taking a longer reflected path
179
What is a light ray that travels down the middle of a fibre called?
Axial ray
180
What is a light ray that doesn't travels down the middle of a fibre called?
Non-axial ray
181
How can modal dispersion be reduced?
By using a single-mode fibre (in which light is only allowed to follow a narrow path)
182
What is material dispersion caused by?
The different amounts of refraction experienced by different wavelengths of light
183
Why is material dispersion a problem?
It causes some of the parts of the signal to take longer to travel down the fibres than others
184
How can material dispersion be stopped?
By using monochromatic light
