Waves Part 1 Flashcards
1
Q
What is a progressive wave?
A
- A progressive wave transfers energy without transferring material.
- The wave is made up of particles of the medium that it is travelling through OR fields oscillating.
2
Q
Example of how you known progressive waves transfer energy?
A
- EM waves are progressive waves and they cause objects to heat up
- X-rays + Gamma rays cause ionisation
3
Q
What is the ‘displacement’ of a vibrating particle of a wave?
A
- The distance and direction (+/-) of the wave from the equilibrium (undisturbed) position.
- Units: m
4
Q
What is the ‘amplitude’ of a wave?
*Look at diagram in notes
A
- The maximum displacement of a wave from the equilibrium position.
- Units: m
5
Q
What is the ‘wavelength’ of a wave?
*Look at diagram in notes
A
- The least distance between two adjacent vibrating particles with the same displacement and velocity at the same time (e.g. the distance between successive peaks)
- Units: m
6
Q
What is the period of a wave?
A
The time taken for one complete wave to pass a fixed point.
Units: s
7
Q
What is the ‘frequency’ of a wave?
A
- The number of complete oscillations passing through a fixed point each second
- Units: Hz
8
Q
What are transverse waves?
A
Waves in which the direction of oscillations (particles or fields) are perpendicular to the direction of energy transfer.
9
Q
Examples of transverse waves?
A
- EM waves
- Secondary seismic waves
- Waves on a string/rope/slinky
- Water ripples
10
Q
What are EM waves?
A
EM waves are waves which consist of electric and magnetic fields oscillating in phase and at right angles to each other and to the direction of energy transfer.
11
Q
What are longitudinal waves?
A
Waves in which the direction of oscillations (particles) are parallel to the direction of energy transfer.
12
Q
What do longitudinal waves consist of?
A
- Longitudinal waves consist of compressions and rarefactions.
- Look at notes to see compressions and rarefactions*
13
Q
Can longitudinal waves travel in a vacuum?
A
Longitudinal waves require a medium in order to travel so unlike transverse which can travel in a vacuum, longitudinal waves cannot.
14
Q
Examples of longitudinal waves?
A
- Sound waves
- Primary seismic waves
15
Q
What is unpolarised light?
A
Unpolarised light is light which consists of waves that are oscillating in more than one plane.
Polarised light is light which consists of waves that are oscillating in only one plane.
16
Q
Unpolarised light can be polarised. What type of waves only can be polarised?
A
Only transverse waves can be polarised. (Longitudinal waves cannot be polarised).
The fact that polarisation is possible is evidence for the nature of transverse waves.
17
Q
What is needed to polarise light waves and what does this do?
A
Unpolarised light can be polarised using a polaroid filter. When unpolarised light passes through this filter, the transmitted light is polarised as the filter only allows light which vibrates in a single direction/plane (according to the alignment of the molecules in the filter) to pass through. The transverse waves are now plane-polarised.
18
Q
What happens when light passes through two polaroid filters at right angles to each other?
A
No light would be transmitted at the other end.
19
Q
When does light become partially polarised?
A
- Light is partially polarised when it is reflected from some surfaces.
- This is such that waves parallel to the plane of incidence are transmitted (at a greater intensity) after reflecting, in comparison to waves in other planes.
*IS THIS CORRECT? If so why? Explain this further.
20
Q
What does partially polarised (after reflection) mean?
A
Light is said to be partially polarized after reflection when one of the planes of the incoming wave is expressed at a greater intensity than the others following reflection. Reflection can cause partial polarisation as the waves which are parallel to the plane-of-incidence are expressed at a higher intensity, compared to the waves in the other planes - other planes do not cancel out, only reflected at a lower intensity.
Fully polarised = other planes completely cancel out.
21
Q
How do polaroid sunglasses make use of the fact that reflected light are partially polarised.
A
- Glare is when light reflects off surfaces into our eyes, blocking our vision.
- The fact that it gets partially polarised reduces the impact of this glare (reduces intensity of light hitting eyes).
- However to completely eliminate the glare, polaroid glasses make use of a polaroid filter with a vertical orientation to allow through the vertically polarised light only (while blocking out any waves in the horizontal plane which is the majority of the partially plane polarised light following refelection).
22
Q
What applications do polarisers have?
A
1) Polaroid material
2) Alignment of aerials for transmission and reception.
23
Q
Why are polarisers important in the alignment of aerials for transmission and reception.
A
TV and radio signals are usually plane-polarised by the orientation of the rods on the transmitting aerial, so the receiving aerial must be aligned in the same plane of polarisation as the rods on the transmitting aerial to receive the signal at full strength.
24
Q
Define wave speed?
A
Distance travelled by the wave per unit time.
Units: m/s
25
For a constant speed, the higher the _______ of a wave, the lower the _______ of the wave.
1) Frequency
2) Wavelength
*Look at EM waves for example, same speed, different frequencies and wavelengths.
26
How to calculate period of a wave?
*Period = time taken for 1 complete wave to pass a fixed point.
Period = 1/f
| ...where f is the frequency of the wave in Hz
27
How to calculate the frequency of the wave given the period?
Frequency = 1/period
28
How to calculate the speed of a wave?
c = fλ
speed of wave(m/s) = frequency(Hz) x wavelength(m)
*c is not the speed of light unless an EM wave, just the wave speed.
29
What is the phase of a vibrating particle?
The phase of a vibrating particle at a certain time is the fraction of the cycle it has completed since the start of the cycle.
Units: radians, degrees or fractions of a cycle/wavelength
30
What is phase difference?
How much a particle/wave lags behind another particle/wave.
Units: radians, degrees or fractions of a cycle/wavelegnth
*If comparing between two waves, these two waves are usually travelling at the same speed.
31
What is the phase difference between two vibrating particles of the same wave that are a wavelength apart?
*Also used to compare between two different waves.
The phase difference is 360°/0° or 2π/0π radians
32
What is the phase difference between two vibrating particles of the same wave that are half a wavelength apart?
*Also used to compare between two different waves.
The phase difference is 180° or π radians
33
What is the phase difference between two vibrating particles of the same wave that are quarter a wavelength apart?
*Also used to compare between two different waves.
The phase difference is 90° or 1/2π radians
34
For two seperate waves, what phase difference do they need to be IN PHASE?
0°, 360° or any multiple of 360°
35
For two seperate waves, what phase difference do they need to be exactly OUT OF PHASE?
180° or any phase differences that are equal to an odd number x 180° (180° x 3, 180° x 5, etc)
36
What do you call waves that are exactly out of phase (out of phase by 180° etc)
The two waves are in antiphase.
37
What would these degrees be in 'fractions of wavelength'?
| 90° 180° 270° 360°
```
90° = 1/4λ
180° = 1/2λ
270° = 3/4λ
360° = 1λ
```
38
What is the phase difference between two vibrating particles of the same wave that are three quarters a wavelength apart?
The phase difference is 270° or 3/2π radians.
39
How to convert fractions of wavelength into degrees or radians?
Into degrees: fraction of wavelength x 360
| Into radians: fraction of wavelength x 2π
40
How to convert degrees to radians and vice versa
degrees x π/180 = radians
| radians / π/180 = degrees
41
What can all waves do?
Waves can refract, reflect and diffract
42
To observe the properties of waves, we can use a ripple tank. What are 'wavefronts' on ripple tanks?
These are lines of constant phase (they highlight the corresponding parts of a wave that are in phase).
43
What direction is the wavefront compared to the direction of wave travel?
The direction of wavefronts is perpendicular to the direction of wave travel
44
In reflection, the angle of incidence is the same as...
...the angle of reflection.
45
What is the angle of incidence?
The angle between the normal and incident wave.
46
What is the angle of reflection.
The angle betweeen the normal and the reflected wave.
47
What is the normal?
The normal is an imaginary line perpendicular to the plane of the surface (where wave will get reflected/refracted)
48
What is refraction?
This is when the direction, speed and wavelength of a wave changes (whilst frequency remains the same) as the wave crosses the boundary to enter a different medium.
49
How does the direction, speed and wavelength of a wave change depending on the angle it hits the boundary?
If the wavefront hits the boundary at an angle the direction, speed and wavelength of the wave changes, however if the wavefront hits the boundary at 90 degrees to it, there is no change in direction however the wavelength and speed will still change.
50
Why does refraction not occur when a wave hits the boundary at 90 degrees to it?
This is because refraction involves a chaneg in direction which does not occur if a wave hits the boundary at 90 degrees.
51
If a wave enters a medium that is more optically dense, at angle what happens to the wave?
- When the wave enters a more optically dense medium:
- speed of wave decreases
- therefore wavelength of wave decreases
- therefore direction of wave changes as it bends towards the normal.
52
What is diffraction?
This is when waves spread out after passing through a gap or around an obstacle.
53
The amount of diffraction depends on?
- Wavelength of the wave ( Longer wavelength = greater diffraction [more spread out])
- Size of the gap (Narrower gap = greater diffraction [more spread out])
54
When is most and least diffraction recieved?
- Most diffraction is achieved when the gap is the same size as the wavelength
- Least diffraction (when diffraction is unnoticeable) is achieved when the gap is bigger than the wavelength.
- When the gap is smaller than the wavelength most waves are reflected.
- If a wave diffracts around an obstacle rather than through a gap, the wider the obstacle compared to the wavelength, theless diffraction.
*The flashcard before suggests that the greatest diffraction is achieved when the gap is narrow as possible and wavelength is long as possible. However here it says when they are same size??
Is the difference betwen the second and third bullet point, that in the 2nd, all the waves still diffract but minimally, but in 3rd, not all waves diffract, and most reflect?
55
How is dish design affected by diffraction?
- Satellite TV dishes in the northern hemisphere need to point south because the satellites orbit the earth directly above the equator.
- The bigger the dish, the stronger the signal it can receive, because more radio waves are reflected by the dish onto an aerial.
- However a bigger dish reflects the radio waves to a smaller focus because it diffracts the waves less.
- The dish therefore needs to be aligned more carefully than a smaller dish, otherwise it will not focus the radio waves onto the aerial.
Dont understand when diffraction is happening? There is no gap or obstacle involved. Just dont understand point 3 and 4 - like why does less diffraction mean smaller focus??
56
What is the principle of superposition?
When two or more waves meet, the resulting displacement is the vector sum of each wave's displacement.
For superposition, for the A-level exam you will focus on coherent waves, but non-coherent waves do this too.
57
What are coherent waves?
Waves are perfectly coherent if their frequency and waveform are identical and their phase difference is constant.
*Phase difference is constant between them does not the phase difference of the two waves is always going to be a certain value like 180°, it just means that the phase difference between them is always the same so it can be 180° at one point for BOTH, then 90° at another point for BOTH..
58
If coherent waves have same frequency and wavelength, what does that tell you about their speed?
Their speed is the same.
59
When two waves superpose, they can interfere with each other in two ways before they move apart. What are they?
When they superpose, they can interfere constructively and destructively.
60
What is constructive interference?
*Dont use word amplitude, rather displacment as amplitude refers to MAXIMUM displacement.
Constructive interference occurs when two wave with displacement in the same direction superpose. The resulting displacement of the superposed wave will be greater than both of the individual waves.
61
What is destructive interference?
*Dont use word amplitude, rather displacment as amplitude refers to MAXIMUM displacement.
Destructive interference occurs when one wave with a positive displacement superposes another wave with a negative displacement. The displacement of the superposed wave will be less than at least one of the waves (or both - total destructive interference)
62
What is maximum/total destructive interference?
This occurs when the two waves with equal but opposite displacements superpose. The displacement of the resulting wave is zero.
*e.g a trough and crest of same disaplacement superpose
63
What phase difference does the two coherent waves require for maximum/total destructive interference?
The two waves need to be 180° out of phase.
64
When a crest meets a crest, what is formed?
Supercrest through constructive interference
65
When a trough meets a trought what is formed?
Supertrough through constructive interference.
66
The resulting amplitude of two waves which cosntructively or destructively superpose and interfere is calculated by?
The sum of the inidividual displacements.
67
What is a stationary wave?
A stationary wave is formed from the superposition of two progressive waves, travelling in opposite directions in the same plane, with the same frequency, wavelength and amplitude.
68
How can you create a stationary wave?
This link shows you how a stationary wave forms: https://www.youtube.com/watch?v=DUPLRe5PHvE
Setup a driving oscillator at one end of a stretched spring. The other end of the string should be fixed in place.
When the oscillator is turned on, a wave will travel down the string and be reflected at the fixed end. When this happens, the original wave will superpose with the reflected wave and because the waves have the same wavelength, frequency and amplitude, a stationary wave will be formed.
The reflected wave and original wave superpose and interfere constructively at fixed positions to produce antinodes, and interfere destructively at other fixed positions to produce nodes.
69
What are nodes?
Points of zero displacement and total destructive interference
70
What are antinodes?
Points of maximum displacement (amplitude) due to constructive interference
71
If the postion of the nodes is the same in a stationary wave even though the position of the antinodes moves from positive to negative displacememnt constantly, what does this tell you about the phase difference between the original and reflected wave at the nodes?
At the nodes, total destructive interference is occuring from the moment a stationary wave is set up. This means at the nodes, the two waves must always be 180°/ π radians out of phase.
72
Key property of stationary waves?
Stationary waves do not transfer energy.
73
The lowest frequency at which a stationary wave occurs is known as?
The first harmonic
74
Describe the first harmonic pattern of vibration?
Stationary wave with two nodes and one antinode.
| Length of string/wave = 1/2λ
75
What is the distance between adjacent nodes in a harmonic?
The distance between adjacent nodes (or antinodes) is half a wavelength (for any harmonic).
76
What happens when the frequency of the first harmonic is doubled?
When the frequency is doubled, the second harmonic pattern of vibration is observed.
77
Describe the second harmonic pattern of vibration?
Stationary wave with three nodes and two antinodes.
| Length of string/wave = 1λ
78
What happens when the frequency of the first harmonic is tripled?
When the frequency is tripled, the third harmonic pattern of vibration is observed..
79
Describe the third harmonic pattern of vibration?
Stationary wave with four nodes and three antinodes.
Length of string/wave = 3/2λ
1st harmonic = 1 antinodes = 2 nodes
2nd harmonic = 2 antinodes = 3 nodes
3rd harmonic = 3 antinodes = 4 nodes
nth harmonic = n nodes = n+1 antinodes
80
To recap: What happens to the displacement of a vibrating particle in a stationary wave?
The displacement of a vibrating particle in the stationary wave varies with position from zero at a node, to max displacement/amplitude at an antinode.
81
When is the phase difference between two vibrating particles in a stationary wave zero degrees (so in phase)?
IMPORTANT!!!
For a stationary wave, the phase difference is 360°/0° (so in phase) if the two particles are between adjacent nodes or seperated by an evene number of nodes (How?)
82
When is the phase difference between two vibrating particles in a stationary wave 180°?
The phase difference is 180° (in antiphase) if the two particles were seperated by an odd number of nodes.
83
Compare the frequency of stationary waves and progressive waves?
For stationary waves, all particles except those at the nodes vibrate at the same frequency.
For progressive waves, all particles vibrate at the same frequency.
*Don't understand.
84
Compare the amplitude of stationary waves and progressive waves?
For stationary waves, the amplitude of vibrating particles varies from zero a the nodes to a maximum at an antinode.
For progressive waves, the amplitude is the same for all particles
*Don't understand.
85
Compare the phase difference between two particles of stationary waves and progressive waves?
For stationary waves, the phase difference is equal to mπ, where m is the number of nodes betweeen the two particles. Between nodes all particles are at the same phase.
For progressive waves, the phase difference is equal to 2πd/λ, where d = distance between particles and λ = the wavelength of the wave.
86
Examples of stationary waves?
- Stationary microwaves can be formed by reflecting a microwave beam at a metal plate, to find the nodes and antinodes use a microwave probe.
- Stationary sound waves can be formed by placing a speaker at one end of a closed glass tube, lay powder across the bottom of the tube, it will be shaken at the antinodes and settle at the nodes. The distance between each node ishalf a wavelength, and the frequency of the signal generator to the speaker is known so by c=fλ the speed of sound in air can be found.
87
We talked about the first harmonic, second harmonic etc.
| What equation can be used to calculate the frequency at which a stationary wave forms is the first harmonic?
f = 1/2L x square root of T/μ
where f = frequency of first harmonic
L = length of vibratn string
T = Tension
μ = mass per unit length of a string.
*How do you calculate the second or third without the first??Does the equstion change?
88
The frequency (of the first harmonic) corresponds to......
*IS IT JUST THE FREQUENCY OF THE FIRST HARMONIC OR SECOND OR WHAT?
...the pitch of a note.
89
The frequency corresponds to the pitch of a note, therefore, the pitch of a note can be changed by?
Changing the frequency of the first harmonic
90
How can we increase the pitch of a note?
To increase the pitch of a note, we need to increase the frequency of the first harmonic:
- Rasing the tension of the string
- Shortening the length of the string
* Vice versa to decrease the pitch
* What about decreasing tthe mass per unit length? Why is this not in the book?
91
How does an oscilloscope wrko?
An electron gun at one end fires electrons through a glass tube onto a fluorescent screen at the other end to produce a waveform. The electron travels through Y-plates first, and then the X-plates.
Pd is applied across the Y-plates to deflect the electrons vertically.
Pd is applied across the X-plates to deflect the electrons horizontally.
92
What is the X-plate connected to?
The X-plate is connected to the oscilloscope's time base circuit.
93
What is the time base circuit?
The time base circuit moves the spot on the fluoresecnt screen (formed by the electrons) left and right to produce the waveform.
94
What units is the X-plate calibrated in?
The X-plate is calibrated in milliseconds per centrimetre (mscm⁻¹). This is because it is connected to the time base circuit which tells you how many milliseconds each cm represents.
* Can be microseconds instead.
* It may not always be given in cm also, so it could be that each division represents a mm.
95
The pd across the Y-plate is determined by the Y-input/Y-sensitivity. What units is the Y-plate calibrated in?
The Y- plate is connected to the Y-input/Y-sensitivity. The Y-input has the units Vcm⁻¹ and this tells you how many volts each cm represents.
*It may not always be given in cm also, so it could be that each division represents a mm.
96
What does an oscilloscope actually do and what is it actually measuring?
FOR ALL OSCILLOSCOPE NEEDS GO HERE: https://www.cyberphysics.co.uk/topics/waves/Oscilloscope/oscilloscope.htm
An oscilloscope plots a voltage against time graph on the screen (a voltmeter which also plots time).
Like a voltmeter connected IN PARALLEL to the component you are observing, measuring the pd through it against time. (It has two ports to connect wire to)
97
The potential difference supplied to the Y-input can be?
AC potential difference
and DC potential difference.
This depends on the pd across the component that the oscilloscope is measuring.
If AC potential difference is supplied to the compoenent being measured, then AC potential difference exists across the Y-plates.
98
In order to produce a waveform on the oscilloscope, what type of pd is needed?
An AC potential difference is required (the time base circuit should also be on).
99
What does the oscilloscope display if AC voltage is supplied and the time base circuit is off?
A vertical line along the y-axis (at a time of 0).
100
What does the oscilloscope display when the Y-input is supplied with DC voltage?
When supplied with DC-voltage and time base circuit is on, produces a horizontal line at the voltage value of the component being measured
When supplied with DC-voltage and time base is off, produces a dot at the voltage value (at the centre).
101
What does the oscilloscope display if no votlage is being supplied to the Y-input?
If time base is on = a straight horizontal line at a voltage value of 0
If time base is off = a dot at a voltage value of 0.
102
What is the peak pd?
pd at the maximum vertical displacement measured from the time-line.
103
What is the peak-to-peak pd?
The difference between the min and max values of pd (most positive minus most negative).
104
How to calculate peak pd?
Peak pd = amplitude of wave (cm) x value of Y-input/Y-sensitivity (Vcm⁻¹)
105
How can you measure the frequency from an oscilloscope waveform?
f = 1/T
| where T is the time for one full cycle
