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AQA A-Level Physics > Chapter 4 - Waves > Flashcards

Chapter 4 - Waves Flashcards

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1
Q

What are mechanical waves?

A
  • Waves that pass through a substance
  • Cannot travel through a vacuum
  • e.g. sound waves, seismic waves, waves on a string
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2
Q

What are mechanical waves made up of?

A

Vibrations passing through the substance

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3
Q

How are sound waves in air created?

A

By making a surface vibrate so it sends compression waves through the surrounding air

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4
Q

What are electromagnetic waves?

A
  • Oscillating electric and magnetic fields that progress through space without the need for a substance
  • Can travel through a vacuum
  • A vibrating electric field generates a vibrating magnetic field, which generates a vibrating electric field further away, and so on
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5
Q

What are longitudinal waves?

A

Waves in which the direction of vibration of the particles is parallel to (along) the direction of energy transfer/direction in which the wave travels

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6
Q

Examples of longitudinal waves

A

Sound waves, primary seismic waves, compression waves along a spring

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7
Q

What are compressions of a longitudinal wave?

A

Areas of high pressure - particles closer together

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8
Q

What are rarefactions of a longitudinal waves?

A

Areas of low pressure - particles further apart

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9
Q

What are transverse waves?

A

Waves in which the direction of vibration is perpendicular to the direction of travel/energy transfer

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10
Q

Examples of transverse waves?

A

Electromagnetic waves, Secondary seismic waves, waves on a string

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11
Q

When are transverse waves plane-polarised?

A

When vibrations stay in one plane (direction) only

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12
Q

When are transverse waves unpolarised?

A
  • When vibrations change from one plane (direction) to another
  • Can vibrate in all directions perpendicular to the direction of propogation (travel)
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13
Q

Why can longitudinal waves not be polarised?

A
  • Particles only vibrate in the same direction that the wave travels
  • Can only travel in one plane, so we do not refer to them as being polarised or unpolarised
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14
Q

How do transverse waves become polarised?

A
  • Unpolarised waves travel through a polariser (e.g. a polaroid filter for light), which only lets vibrations in a certain direction through, according to the alignment of its molecules
  • Polariser blocks waves of other planes/polarisations so they cannot pass through
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15
Q

What happens when light passes through two polaroid filters at 0°/180°/360° to each other

A

Light intensity at a maximum (that it can be when polarised, but less than if the light was unpolarised)

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16
Q

What happens when a second polaroid filter is rotated from being in alignment with a first filter to being at 90° to it

A
  • Light intensity reduces from a maximum as the second filter is rotated
  • Intensity at a minimum when the second filter reaches 90°
  • First filter polarises light in one plane, second filter polarises light at a right angle to this plane, so now a minimum amount can pass through
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17
Q

What happens when a second polaroid filter is rotated form being at 90° to a first filter to being in alignment with it

A
  • Light intensity increases from a minimum as the second filter is rotated
  • Intensity at a maximum when the second filter is aligned wioth the first
  • Both filters polarising the light in the same plane, so second filter has no overall effect on intensity and it remains at a maximum
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18
Q

What is the plane of polarisation of an electromagnetic wave?

A

The direction in which the electric field oscillates

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19
Q

What effect do polaroid sunglasses have on light?

A
  • Reduce the glare of light reflecting off of water or glass
  • Reflected light is polarised and intensity reduced when it passes through the polaroid sunglasses
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20
Q

What is the displacement of a wave/vibrating particle?

A

Its distance and direction from its equilibrium position

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21
Q

What is the amplitude of a wave?

A

The maximum displacement of a vibrating particle:
- Height or depth of a transverse wave from its equilibrium position
- Distance between particles in areas of compression (the denser the compression, the higher the amplitude)

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22
Q

What is the wavelength of a wave?

A

The least distance between two adjacent vibrating particles with the same displacement and velocity at the same time
- Distance between adjacent crests of a transverse wave
- Distance between adjacent compressions of a longitudinal wave

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23
Q

What is one complete cycle of a wave?

A

Motion of a wave from one maximum displacement to the next maximum displacement
- From one wave peak to the next
- Wavelength is the distance of one complete cycle

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24
Q

What is the period of a wave?

A

The time for one complete wave to pass a fixed point

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25
Q

What is the frequency of a wave?

A
  • The number of complete waves passing a point per second
  • The number of cycles of vibration of a particle per second
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26
Q

Period equation

A

Period = 1/frequency

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27
Q

What is the relationship between frequency and wavelength?

A
  • Wavelength and frequency are inversely proportional
  • The higher the frequency of a wave, the shorter its lenght, and vice versa
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28
Q

Wavespeed equation

A

Wavespeed, c = fλ
- Each wave crest travels a distance equal to one wavelength, λ, in the time taken for one cycle
- The time taken for one cycle = 1/f
- Therefore wavespeed = distance travelled/time taken = λ/1/f = λf

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29
Q

What is the definition of the phase of a particle?

A

The phase of a vibrating particle at a certain time is the fraction of a cycle it has completed since the start of a cycle

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30
Q

What is the phase difference of two particles?

A

Phase difference between two vibrating particles at the same frequency is the fraction of a cycle between the vibrations of the two cycles
- Measured in either degrees or radians, where 1 cycle = 360° = 2π radians

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31
Q

How can different properties of waves be observed?

A

Using a ripple tank - a shallow transparent tray of water with sloping sides

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32
Q

Why are the sides of a ripple tank sloped?

A
  • To prevent waves reflecting off the sides of the tank
  • If they did reflect, it would be difficult to see the waves
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33
Q

How does a ripple tank work?

A
  • A pattern of wavefronts of constant phase difference (e.g. crests) are cast on the floor
  • The direction in which the wave travels is at roght angles to the wavefront
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34
Q

Why can wavefronts be observed on the screen of a ripple tank?

A
  • Each wave crest acts like a convex lens and concentrates the light onto the screen so the pattern on the screen shows the wave crests
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35
Q

What is reflection?

A

When straight waves are directed at a certain angle to a hard flat surface (the reflector), and reflected off at the same angle

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36
Q

Relationship between the angles of the reflected and incident rays to the normal

A
  • The angle between the reflected wavefront and the surface is the same as the angle between the incident wavefront and the surface
  • The direction of the reflected wave is at the same angle to the reflector as the direction of the incident wave
  • Effect observed when the light ray is directed at a plane mirror
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37
Q

What is refraction?

A
  • When waves pass across a boundary causing the wavespeed and wavelength to change
  • If the wavefronts approach at an angle to the boundary, they change direction as well as speed
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38
Q

Effect of refraction on a wave when passing from a more dense to a less dense material

A

E.g. a light wave travelling from from glass to air:
- Wave travels more slowly in glass than in air
- Wavelength smaller as it’s travelling slower
- Wavelength shortening while crossing at an angle causes wave to change direction and bend away from the normal

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39
Q

What is diffraction?

A
  • Occurs when waves spread out after passing through a gap or round an obstacle
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40
Q

How does the width of the gap affect the amount of diffraction?

A

The narrower the gap, the more the waves spread out

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41
Q

How does wavelength affect the amount of diffraction?

A

The longer the wavelength, the more the waves spread out

42
Q

Why does diffraction happen?

A
  • Consider each point on a wavefront as a secondary emitter of wavelets
  • The wavelets travel only in the direction that the wave is travelling, not in the reverse direction
  • Wavelets combine to form a new wavefront spreading beyonf the gap
43
Q

Equation for phase difference

A

The phase difference in radians = 2πd/λ

44
Q

Why does a bigger satellite dish need to be aligned more carefully than a smaller one

A
  • Satellite TV dishes in Europe must face south as satellites orbit the Earth above the equator
  • The bigger the dish the stronger the signal it can receive
  • A bigger dish means more radio waves are reflected by the dish onto the aerial
  • But a bigger dish reflects the waves to a smaller focus because it diffracts the waves less
  • The dish therefore needs to be aligned more carefully than a smaller one, otherwise it will not focus the radio waves onto the aerial
45
Q

Definition of superposititon

A

When two waves meet, the total displacement at a point is equal to the sum of the individual displacements at that point

46
Q

Effect of superposition when a crest meets a crest

A
  • A supercrest is created
  • The two waves reinforce each other
47
Q

Effect of superposition when a trough meets a trough

A
  • A supertrough is created
  • The two waves reinforce each other
48
Q

Effect of superposition when a crest meets a trough

A
  • The resultant displacement is zero
  • Waves cancel each other out
  • If they are not at the same amplitude, the resultant is a minimum
49
Q

Examples of superposition

A
  • Stationary waves on a rope
  • Water waves in a ripple tank
50
Q

Superposition of stationary waves on a rope

A
  • Two progressive waves of the same frequency sent continuously along a rope from either end
  • Waves superpose at fixed points to form nodes
51
Q

What is a node on a stationary wave?

A
  • Points of no displacement
  • At each node, the two sets of waves are always 180° out of phase, so they cancel each other out
52
Q

What are coherent waves?

A
  • Waves vibrating with the same frequency with a constant phase difference
53
Q

How are nodes formed on a stationary wave?

A
  • At each node, the two sets of waves are always 180° out of phase, so they cancel each other out through superposition
54
Q

What is a progressive wave?

A

A wave which travels continuosly in a medium in the same direction without a change in its amplitude

55
Q

Superposition of water waves in a ripple tank

A
  • A vibrating dipper on a water surface sends out circular waves
  • Points of cancellation (constructive interference) when a crest from one dipper meets a trough from the other - seen as gaps in the wavefronts
  • Points of reinforcement (destructive interference) where a crest from one dipper meets a crest from the other, or a trough from one dipper meets a trough from the other
56
Q

What is interference?

A
  • Waves continuously pass through each other at constant frequency and phase difference - waves are coherent
  • Cancellation and reinforcement happen at fixed positions
57
Q

What is constructive interference?

A
  • When two waves are travelling in the same direction and in phase with each other
  • Superposition occurs and the amplitude of the resultant wave is the sum of the individual waves
58
Q

What is destructive interference?

A
  • When two waves are travelling in the same direction and 180° out of phase
  • Superposition occurs and the two amplitudes cancel each other out
59
Q

What would be the effect on interference if the phase difference of the two waves changed at random?

A
  • Points of cancellation and reinforcement (interference) would move about at random
  • Therefore no interference pattern would be seen
60
Q

How can a microwave transmitter be used to show that microwaves become weaker as they travel?

A
  • Place a receiver in the path of a microwave beam from the transmitter
  • Move the receiver gradually away from the transmitter
  • The receiver signal decreases with distance from the transmitter, so microwaves become weaker as they move away from the transmitter
61
Q

How can a microwave transmitter be used to show that microwaves cannot pass through metal?

A
  • Place a metal plate in between a microwave transmitter and the receiver
  • Receiver will have no signal, so microwaves cannot pass through the metal
62
Q

How can a microwave transmitter be used to show that microwaves diffract?

A
  • Use two metal plates to make a narrow slit in the path of the microwave beam, between the transmitter and receiver
  • Move the receiver in a curve/semicircle
  • Reading drops when close to the plates at either end
  • Reading increases when moved towards the centre of the curve parallel to the gap
  • Keep reader in fixed position and widen the gap to see the reading drop as the waves diffract less
63
Q

How can a microwave transmitter be used to show that microwaves interfere?

A
  • Use two metal plates to make a pair of slits in the path of the microwave beam, between the transmitter and receiver
  • Move the receiver along the metal plates from a distance to find points of maxima and minima where constructive and destructive interference has taken place
64
Q

What is the first harmonic/fundamental mode of vibration?

A
  • A stationary wave of a single loop with a node at either end (two in total)
65
Q

What is an antinode?

A
  • The point with maximum amplitude halfway between two nodes
66
Q

Equation for the distance between adjacent nodes

A

Distance between adjacent nodes = 1/2λ

67
Q

When is the second harmonic formed?

A
  • When the frequency is twice as high as it is to form the first harmonic, corresponding to half the previous wavelength
  • Lenght of the rope now one full wavelength
  • Frequency at each harmonic is an integer multiple of the frequency at the first harmonic
68
Q

How do stationary waves vibrating freely not transfer energy to surroundings?

A
  • Amplitude of vibration is zero at the nodes so there is no energy
  • Amplitude of vibration is maximum at the antinodes, so there is maximum energy
  • Nodes and antinodes at fixed positions, so no energy is transferred in a freely vibrating stationary wave pattern
69
Q

How do stationary waves form?

A

Two progressive waves pass through each other with the same frequency
- Usuall one progressive wave and its reflection

70
Q

Cycle of a stationary wave

A
  • When in phase, they interfere constructively and produce a larger wave
  • 1/4 of a cycle later, the waves have moved 1/4λ in opposite directions so are now in antiphase and interfere destructively to cancel each other out
  • A further 1/4 cycle later, they are in phase again so interfere constructively, but reversed (i.e. if first was two crests meeting, it is now two troughs meeting)
71
Q

Position of nodes

A

Nodes in fixed posititons with the stationary wave oscillating between them

72
Q

When is the phase difference between two vibrating particles on a stationary wave zero?

A

When the two particles are between adjacent nodes or separated by an even number of nodes

73
Q

When is the phase difference between two vibrating particles on a stationary wave 180°/π

A

When the two particles are separated by an odd number of nodes

74
Q

How does the frequency of vibrating particles in a stationary wave vary?

A

All particles except those at nodes vibrate at the same frequency

75
Q

How does the frequency of vibrating particles in a progressive wave vary?

A

All particles vibrate at the same frequency

76
Q

How does the amplitude of vibrating particles in a stationary wave vary?

A

Varies from zero at the nodes to a maximum at the antinodes

77
Q

How does the amplitude of vibrating particles in a progressive wave vary?

A

Amplitude the same for all particles

78
Q

How does the phase difference between two particles in a stationary wave vary?

A

Equal to mπ, where m is the number of nodes between the particles

79
Q

How does the phase difference between two particles in a progressive wave vary?

A

Equal to 2πd/λ

80
Q

How can a microwave transmitter be used to show a stationary wave?

A
  • Direct microwaves from a transmitter normally at a metal plate
  • Metal plate reflects microwaves back at the transmitter, which form a stationary wave with waves from the transmitter
  • Move the detector along the line between the transmitter and the metal plate
  • Detector signal zero or a minimum at equally spaced positions along the line where nodes occur, spaced out at intervals of half a wavelength
81
Q

Displacement when two troughs meet

A

Superpose to produce a maximum negative displacement, not a minimum

82
Q

Controlled arrangement to produce stationary waves (required practical)

A
  • Tie one end of a string to a mechanical vibrator connected to a frequency generator
  • Pass the other end of the string over a pulley, supported by a weight which keeps tension constant
  • Increase the frequency on the generator to produce different patterns of stationary waves
83
Q

When is the first harmonic pattern of vibration seen?

A

At the lowest possible frequency that gives a pattern
- Length, L = 1/2λ1, so λ1 = 2L
- Therefore first harmonic frequency, f = c/λ1 = c/2L

84
Q

When is the second harmonic seen?

A
  • Node in the middle with 2 loops, so string has 2 loops
  • Wavelength of the stationary wave, λ2 = L as each loop is 1/2λ
  • Therefore equation for frequency of the second harmonic, f2 = c/λ2 = c/L = 2f1
85
Q

At what frequencies do stationary wave patterns appear?

A

nf1, where n is the nth harmonic

86
Q

How do stationary waves form?

A
  • A progressive wave is sent out by a vibrator
  • The crest reverses its phase when it reflects at the fixed end and travels back as a trough
  • When reaching the vibrator, it reflects and reverses phase again, travelling away as a crest
  • This is reinforced by a new crest from the vibrator so the amplitude increases
87
Q

Conditions for a stationary wave to form

A
  • The time taken for a wave to travel along the string and back should be equal to the time taken for a whole number of cycles of the vibrator
  • Time taken for wave to travel along string and back = 2L/c
  • Time taken for vibrator to pass through a whole number of cycles = m/f, where m is an integer
    2L/c = m/f
    L = mλ/2 = a whole number of half wavelengths
88
Q

What does the pitch of a note correspond to?

A

Frequency

89
Q

How can the pitch of the note from a stretched string be altered?

A

By changing the tension or altering the length of the string

90
Q

How can the pitch of the note from a stretched string be increased?

A

Raising the tension or shortening the length

91
Q

How can the pitch of the note from a stretched string be decreased?

A

Lowering the tension or increasing the length

92
Q

Equation linking frequency and tension

A

4f = 1/2l√T/μ

  • f = first harmonic frequency
  • T = tension
  • µ = mass per unit length
93
Q

How to use a tuning fork to tune a vibrating string

A
  • By changing the length or altering tension
  • Sound from vibrating string includes all harmonic frequencies, tuning fork vibrates only at a single frequency
  • Wire tuned when its first harmonic frequency is the same as the tuning fork frequency
94
Q

How to check is a wire is tuned?

A
  • Balance a small piece of paper on the wire at its centre
  • Place the vibrating tuning fork at one end of the wire
  • Paper will fall off the wire if it is tuned correctly
95
Q

How does an oscilloscope work?

A
  • A filament emits electrons in a beam into an electron tube towards a flourescent screen at the other end of the tube
  • Light is emitted from the spot on the screen where the beam hits the screen
96
Q

How is the spot on an oscilloscope screen moved?

A

Affected by pd across either pair of deflecting plates:
- no pd across either and spot stays in same position
- deflected vertically if pd is applied across the Y-plates
- deflected horizontally if pd is applied across the X-plates
- Displacement proportional to applied pd in both cases

97
Q

What are the X-plates of an oscilloscope used for?

A
  • X-plates connected to time base circuit
  • Makes spot move at constant speed left to right across the screen, then back again much faster
  • As spot moved at a constant speed, the x-scale can be calibrated, usually in milliseconds or microsecond per cm
98
Q

What are the Y-plates of an oscilloscope used for?

A
  • Pd to be displayed connected to Y-plates via the Y-input
  • Spot moves up and down as it moves left to right
  • Traces out waveform on screen
  • Vertical displacement of spot proportional to pd applied to the Y-plates, so Y-input calibrated in volts per cm
99
Q

How to measure peak pd from an oscilloscope

A
  • Measure the amplitude of the waveform from the y-axis
  • Multiply amplitude by set value of the Y-gain from the dial
100
Q

How to measure frequency of the alternating pd from an oscilloscope

A
  • Measure the distance for one full cycle horziontally across the x-axis
  • Multiply distance for one full cycle by set time base from the dial to find time period (time for one full cycle)
  • frequency = 1/T

AQA A-Level Physics (6 decks)

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