Unit 2: Waves Flashcards
1
Q
When a stretched spring is vibrated at one end, what form of wave are the oscillations along the spring transferred in?
A
In the form of travelling or progressive waves.
2
Q
What are some of the features of progressive waves in terms of a spring?
A
They transfer energy from one end of the spring to the other, but the coils of the spring are not permanently displaced by the wave - the coils simply oscillate from side to side before returning to their equilibrium positions.
3
Q
In what way do mechanical waves travel?
A
Repeated oscillations of the particle are transferred through the medium.
4
Q
What is the amplitude of a wave?
A
It is the maximum displacement from equilibrium caused by the wave.
5
Q
What is an example of a mechanical wave travelling through a medium and the effects it has?
A
A sound wave travels through air as the air molecules vibrate backwards and forwards.
6
Q
What does the speed of a mechanical wave depend on?
A
The properties of the medium that it is travelling through - in particular the elasticity of the medium and the inertia of the medium .
7
Q
What is the elasticity of the medium?
A
This is the strength of the forces between adjacent particles.
8
Q
What is the inertia of a medium?
A
The resistance to acceleration.
9
Q
What is wave speed measured in?
A
m/s
10
Q
Will a wave travel quickly in a spring that has a high spring constant and why?
A
Yes as it will cause large forces and the wave will travel quickly.
11
Q
Will a wave travel quickly in a spring that has a high spring constant and why?
A
Yes as it will cause large forces and the wave will travel quickly.
12
Q
What is the frequency of a wave determined by?
A
The frequency of the oscillations that caused it.
13
Q
What is the frequency of a wave?
A
It is the number of cycles that occur in one second.
14
Q
What is the unit for frequency?
A
Hz
15
Q
What does a frequency of 1Hz represent?
A
It is one wave per second.
16
Q
What is the wavelength?
A
It is the distance between identical consecutive points on a wave.
17
Q
What is the formula for wave speed?
A
c = fλ
18
Q
When are two points in phase?
A
When the two points are a whole wavelength apart and are oscillating in time with each other.
19
Q
What is the formula for the phase difference in degrees?
A
φ = 360 x (x1 - x2/λ)
20
Q
What is the difference in phase between two points on the same wave expressed as?
A
An angle - φ
21
Q
What is the path difference?
A
The difference between the distances travelled by two waves that originally had the same wavelength and they were in phase.
22
Q
What is the formula for the phase difference in degrees?
A
φ = 360 x (x1 - x2/λ)
23
Q
What is the path difference?
A
The difference between the distances travelled by two waves that originally had the same wavelength and they were in phase.
24
Q
What is the equation of the phase difference in terms of the path difference in radians?
A
phase difference = 2π x (path difference/λ)
25
What are the vibrations of a longitudinal wave like?
Longitudinal waves have vibrations that are parallel to the direction in which the wave is travelling.
26
What are some examples of longitudinal waves?
Sound waves, seismic P-waves and compression waves in a spring.
27
What are compressions in terms of longitudinal waves?
They are regions of higher pressure and density.
28
What are rarefactions in terms of longitudinal waves?
They are regions of lower density and pressure.
29
How can compressions and rarefactions be caused?
Through moving the source backwards and forwards.
30
What are the vibrations like of transverse waves?
They have vibrations that are perpendicular to the direction of propagation.
31
What are some examples of transverse waves?
Water waves, waves on strings, seismic S-waves and electromagnetic waves.
32
What is different about EM waves in comparison to the other transverse waves?
They are not vibrations of particles in a medium but are oscillating electric and magnetic fields.
33
What characteristics do the oscillating electric and magnetic fields of an EM wave have?
They are perpendicular to each other and the direction of travel.
34
What is the speed of an electromagnetic wave?
3 x 10^8 m/s
35
What is the speed of an electromagnetic wave?
3 x 10^8 m/s
36
In what plane do transverse waves have oscillations in?
A plane that is perpendicular to the wave's velocity.
37
In what direction could the oscillations of a wave occur in a given plane?
Any direction
38
What is an unpolarised wave?
A wave that causes oscillations in any direction in that plane.
39
When is a wave said to be polarised?
When the oscillations in a plane are restricted so that they travel in one direction only.
40
How are polarised EM waves used in television signals?
The radio waves that are used to carry TV signals are transmitted as horizontally or vertically polarised waves and the TV aerial must be aligned in the same plane in order to get the signal.
41
How can polarised light be produced?
By passing the light through a sheet of Polaroid.
42
What is the effect of two sheets of Polaroid being at right angles to each other?
They will block off all the light.
43
What does the fact that light can be polarised provide evidence for?
It suggests that light travels as a transverse wave.
44
What are 'optically active' materials?
Those that can twist the direction of the polarisation of light.
45
What does the angle through which the direction of polarisation is twisted depend on for optical active materials and hence what can this be used to examine?
It depends on the stress which the material is under which means that it can be used to examine stress patterns.
46
What can change the angle of polarisation?
An applied electric field.
47
What is refraction?
When a wave moves from one medium into another and changes direction due to a change in speed.
48
What happens to the wavelength, the frequency and the wave speed of a wave as it travels from air into glass?
The speed and the wavelength both decrease but the frequency remains the same.
49
What happens if the wavefront hits the boundary between the two media at an angle?
Then the wavefront changes direction.
50
What does a ray of light do when it slows down?
It refracts towards the normal.
51
What does a ray of light do when it slows down in terms of the normal?
It refracts towards the normal.
52
What does a ray of light do when it speeds up in terms of the normal?
It refracts away from the normal.
53
What will cause a ray of light to deviate more from its original path?
A greater change in speed.
54
What is the refractive index of air and what assumption does this lead to?
1 which means that light travels at the same speed in air as in the vacuum.
55
What is the refractive index of a material?
It is the ratio of the speed of light in a vacuum to its speed in the material.
56
What is the formula for the refractive index of a material and what is this sometimes referred to as?
n = speed of light in a vacuum / speed of light in the material, this is sometimes referred to as the absolute refractive index.
57
What is the normal?
The line drawn at right angles to the boundary.
58
Why is the value of the absolute refractive index always greater than one?
Light always travels faster in a vacuum than in any other material.
59
Why does the absolute refractive index have no unit?
It is a ratio
60
What does a high refractive index mean?
It means that light is slowed down more and it is deflected more.
61
What are materials that have a high value of refractive index said to be?
Optically dense
62
What is the formula for the relative refractive index?
1n2 = speed of light in medium 1 / speed of light in medium 2
63
What formula can be used to calculate the relative refractive index for a light wave moving between two media?
1n2 = n2 / n1 where n2 and n1 are the absolute refractive indices of materials 2 and 1.
64
What is the relative refractive index of a wave moving from medium 2 to medium 1?
It is the inverse of the relative refractive index for a wave moving in the opposite direction from medium 1 to medium 2.
65
Can the relative refractive index have a value of less than 1?
Yes
66
What is snell's law?
n1sinθ1 = n2sinθ2
67
What is snell's law?
n1sinθ1 = n2sinθ2
68
What is the effect on light and the angles of incidence and refraction if it travels into an optically denser material?
It will slow down and θ2 will be less than θ1.
69
What does snell's law apply to?
When a light ray passes from one medium to another, for monochromatic light and rays which lie in the same plane.
70
What is monochromatic light?
Light that has a single wavelength.
71
What are adjacent wavefronts separated by?
One wavelength.
72
What is total internal reflection?
When a ray of light leaving an optically dense material and travelling into a less dense one is not refracted out of the dense material but is totally reflected back inside.
73
When does total internal reflection occur?
When the light ray is moving from one medium into another in which the speed of light is greater and the angle of incidence is greater than the critical angle.
74
When the incidence angle is smaller than the critical angle, what happens?
There is partial reflection.
75
Why is snell's law not applicable in total internal reflection?
The sine of an angle cannot be greater than 1 and the ray is no longer refracted.
76
Why is snell's law not applicable in total internal reflection?
The sine of an angle cannot be greater than 1 and the ray is no longer refracted.
77
Why is the change from refraction to reflection not sudden?
Some light is always reflected inside the block.
78
What happens to the amount of light that is reflected back inside the block as the angle of incidence increases?
More and more light is reflected
79
What happens at the critical angle?
The refracted ray disappears - at this angle the ray is trying to travel along the boundary between the two materials.
80
What happens if the angle of incidence is greater than the critical angle?
All the incident light is reflected and none of it escapes.
81
What does the value of the critical angle depend on?
The refractive indices of the two other media.
82
What is the formula for the critical angle?
sinθc = n1 / n2
83
What is the formula for the critical angle?
sinθc = n1 / n2 OR 1/n
84
Why is a reflection clearer when a prism is used rather than a mirror?
With a mirror, silvering causes multiple reflections.
85
When are totally reflecting triangular prisms used?
In binoculars to invert the image.
86
What is total internal reflection responsible for?
Making precious stones sparkle and the mirage which makes a tarmac road appear shiny on a hot day.
87
What is the most important application of total internal reflection?
In optical fibres
88
What are optical fibres used for?
They carry cable TV and telephone communications and they are the backbone of computer networks.
89
How does an optical fibre work?
It is a thin strand of pure glass and light is refracted into one end and strikes the internal wall of the fibre at an angle of incidence greater than the critical angle. Total internal reflection occurs and the light is confined within the fibre.
90
Why do light losses occur?
Optical fibres are used to carry information in the form of digital pulses of infrared light over long distances.
91
How can low levels of light losses be achieved?
By using glass with a very low level of impurities since impurity atoms can scatter the light so that it strikes the boundary at less than the critical value and is refracted out of the fibre.
92
What is a potential source of signal loss?
Scratches on the surface of the fibre.
93
What is a way in which the potential source of signal loss can be avoided?
By using cladding to protect the inner core of the fibre.
94
What is cladding?
An outer layer of different glass.
95
What are the properties of cladding and why?
It is made of glass of a lower refractive index than the core otherwise total internal reflection could not take place.
96
What is the bandwidth?
The number of digital pulses that can be transmitted per second along a fibre.
97
Do optical fibres have a greater bandwidth than conventional copper cables?
Yes
98
What is the critical angle of an optical fibre?
sinθc = refractive index of the cladding / refractive index of the core
99
What is a way in which the potential source of signal loss can be avoided?
By using cladding to protect the inner core of the fibre as scratches in the cladding do not matter because it is not carrying the light the light signal.
100
What is the critical angle of an optical fibre?
sinθc = refractive index of the cladding / refractive index of the core
101
What are endoscopes?
Optical fibres that are used in medicine to help doctors see inside the body - they consist of a bundle of optical fibres used to carry light down into the body and then another bundle of optical fibres is used to carry the image information back to the video camera.
102
What are endoscopes?
Optical fibres that are used in medicine to help doctors see inside the body - they consist of a bundle of optical fibres used to carry light down into the body and then another bundle of optical fibres is used to carry the image information back to the video camera.
103
What does the resultant wave depend on when two waves meet?
The amplitude and the relative phase of the two waves.
104
What is the resultant of waves that are in phase?
They add together constructively.
105
What is the resultant of waves that are in antiphase?
They add destructively.
106
What is the principle of superposition?
The resultant displacement caused by two waves arriving at a point is the vector sum of the displacements caused by each wave at that instant.
107
When can a stationary wave form?
When two continuous similar waves are travelling in opposite directions and they superpose.
108
How is a stationary wave different from a progressive wave?
A stationary wave is a fixed pattern of vibration - no energy is transferred along the wave.
109
What is a synonym of a stationary wave?
A standing wave
110
What can be said about the amplitude of a progressive wave that can't about a standing wave?
Each point on the wave has the same amplitude.
111
What does the amplitude of a point depend on in a standing wave?
It depends on its position.
112
What are nodes?
The points on a standing wave which do not vibrate at all.
113
What is happening at nodes?
The waves travelling in opposite directions always add together to give zero displacement.
114
What is the distance between two nodes?
Half a wavelength
115
What occurs between the nodes?
Antinodes
116
What occurs at antinodes?
Positions of maximum displacement.
117
What occurs at antinodes?
Positions of maximum displacement.
118
Do the points between two nodes vibrate in phase?
Yes
119
When do points in a stationary wave vibrate in antiphase?
When the points are in adjacent loops.
120
What happens to the phase difference in a progressive wave?
The phase changes in with position along the wave.
121
What are the source of vibrations for musical notes?
Stationary waves on the string of an instrument.
122
How are stationary waves caused on a string of a musical instrument?
By being plucked or scraped with a bow, reflections from either end superpose to cause the stationary wave.
123
What occurs at the end of the strings and why?
Nodes as the string is fixed at both ends.
124
What is the simplest way in which a string can vibrate and what is this wave pattern known as?
With one antinode in the middle of the string and this wave pattern is known as the fundamental mode.
125
What is the frequency of the fundamental mode known as?
The fundamental frequency.
126
What is the frequency of the fundamental mode known as?
The fundamental frequency.
127
How can the fundamental frequency be calculated?
fo = c / λ
128
How can the fundamental frequency be calculated?
fo = c / 2l where l is the length of the string
129
What is the first overtone?
When a string can support oscillations which have a node in the centre of the string.
130
What is the frequency for the first overtone?
f1 = c/l where l is the length of the string
131
What is the frequency for the first overtone?
f1 = c/l where l is the length of the string
132
What happens if two waves meet exactly in phase?
They will add together constructively to form a double-height wave.
133
What happens if the waves are exactly out of phase?
They will add together destructively and cancel each other out.
134
Why do we never observe two light waves adding together to give darkness?
The phase difference between two waves is always changing.
135
What needs to happen in order to observe a steady interference pattern?
We need to have two waves that maintain a fixed phase difference over a period of time.
136
When are wave sources coherent?
When two wave sources emit waves that are of constant wavelength which always have the same phase difference.
137
How can two loudspeakers produce coherent wave sources?
Two identical loudspeakers driven by the same signal generator will produce coherent wave sources.
138
What happens when two coherent sound waves from two different sources meet at a point which is equidistance from the two sources?
There will be a loud region as the path difference between the two waves will be any whole number of wave lengths.
139
When will a minimum occur?
When the path difference is one half of a wavelength.
140
When will a maximum occur?
When the path difference is nλ
141
When will a minimum occur?
When the path difference is (2n+1)λ/2
142
When will a minimum occur?
When the path difference is (2n+1)λ/2
143
Why is it very difficult to produce coherent light sources?
Light is emitted from atoms as a result of energy changes in their electrons, these emissions are random and give rise to pulses of light that last for a very short time, these pulses of light have random phase and random polarisation.
144
Why can monochromatic light sources not be coherent sources?
The sources would not have a fixed phase relationship.
145
What was the setup of Thomas Young's experiment?
He used a single light source and passed the light source through a narrow slit, and then used two slits to divide the wavefront coming from the single slit.
146
What do the two slits act as?
The two slits then act as two sources.
147
What do the two slits act as?
The two slits then act as two sources.
148
Why must the two slits be two sources of coherent light?
These sources emit light which originally came from the same wave so they must be coherent.
149
What was the pattern observed on the screen from Young's experiment?
A interference pattern that consisted of a series of equally spaced light and dark bands.
150
What does Young's double slit interference prove?
The wave theory of light.
151
What does the fringe separation (w) depend on?
The distance between the two slits (s), the wavelength of light (λ) and the distance between the slits and the screen (D).
152
What is the formula for fringe separation?
w = λD / s
153
What does increasing the distance between the two slits do to the interference fringes?
It makes the interference fringes closer together.
154
What does reducing the wavelength do to the interference pattern?
The fringes will be closer together.
155
What happens if we increase the distance between the slits and the screen?
The fringes will be further apart.
156
Why can laser light damage eyesight?
As the beam does not diverge, the power is concentrated into a small area which can damage the sensitive cells on the retina of the eye.
157
How is laser light different to normal light?
It is monochromatic, it is coherent and they are highly directional.
158
How is laser light different to normal light?
It is monochromatic, it is coherent and they are highly directional.
159
What is diffraction?
The spreading of waves as they pass through a gap or travel past an obstacle.
160
What does the amount of diffraction depend on?
The relative size of the wavelength and the gap.
161
When is diffraction noticeable?
When the wavelength and gap size are similar.
162
What is the diffraction pattern from a single slit?
A series of bright and dark fringes, there is a broad, bright central maximum with narrower, less bright, secondary maxima on either side
163
What happens to the central maximum if there is more diffraction?
It gets wider
164
What causes the central maximum to get wider?
If the wavelength is longer and the gap is smaller.
165
What causes the central maximum to get wider?
If the wavelength is longer and the gap is smaller.
166
Why is blue light diffracted less than red light for a particular slit size?
It has a shorter wavelength.
167
What is the effect of the single slit diffraction on the double slit interference pattern?
The single slit diffraction pattern is superimposed on the two-slit diffraction pattern so the intensity of the interference maxima is limited by the diffraction pattern of the single slit.
168
What happens as you increase the number of slits?
The interference maxima get sharper and the maxima get further apart.
169
What is a diffraction grating?
A series of narrow, parallel slits usually formed by ruling lines on glass.
170
What needs to be the case for a bright fringe to occur?
dsinθ = nλ
171
How can you derive the formula for a bright fringe?
Consider monochromatic light that strikes a diffraction grating at right angles. A and B are identical points in adjacent slits separated by slit separation, d. The path difference between the light from A and B in the same direction θ is therefore dsinθ. At some angles the path difference will equal a whole number of wavelengths - all the light in these directions will be in phase and there will be an interference maximum.
172
When will the first maximum occur and what is this known as?
When n = 0 i.e. light from adjacent slits has zero path difference and this is known as the zero-order maximum.
173
What needs to be the case for a bright fringe to occur?
dsinθ = nλ
174
When will the first maximum occur and what is this known as?
When n = 0 i.e. light from adjacent slits has zero path difference and this is known as the zero-order maximum.
175
What happens at the first-order maximum?
The light from adjacent slits has a path difference of exactly one wavelength and so all the light waves are in phase.
176
Where are the two first order maxima positioned?
Symmetrically either side of the straight-through beam.
177
What is the highest order diffraction that can be observed and why?
n = d / λ as the largest possible value for sinθ is 1.
178
How can a spectrometer be used to measure the wavelength of light?
The spectrometer uses a collimator to produce parallel light which illuminates a diffraction grating at right angles, the resulting diffraction pattern is observed through a small telescope.
179
How can you calculate d when you are given the number of line per metre, N?
d = 1 / N
180
What is the interference pattern of white light when it is shined onto a diffraction gradient?
The straight-through beam will be white as all waves will arrive in phase, the shortest wavelengths will have an interference maximum at the smallest angle so we would expect to see blue and violet maxima first followed by longer wavelengths up to red at largest angles.
181
What effect does a diffraction grating have?
It splits light up into its various colours so each order now becomes a spectrum.
182
What effect does a diffraction grating have?
It splits light up into its various colours so each order now becomes a spectrum.
183
Why are diffraction gratings useful for analysing the light from stars?
They determine what elements are present, what the temperature of the star is and even how fast the star is moving.
