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Physics A level > Section 3 - Waves > Flashcards

Section 3 - Waves Flashcards

1
Q

Wave def

A

The oscillation of particles of fields

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2
Q

Progressive wave def

A

Moving wave - carries energy from one place to another without transferring any material

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3
Q

Evidence of how waves carry energy

A
  • EM waves cause things to heat up
  • X rays and gamma rays cause ionisation
  • Sounds cause vibrations
  • Wave power can be used to generate electricity
  • Since waves carry energy away, the source loses energy
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4
Q

Wave parts diagram

A

INSERT PIC

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5
Q

Wave Cycle

A

One complete vibration of the wave

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6
Q

Wave Displacement

A

X in metres - how far from a point a wave has moved from its undisturbed position

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7
Q

Wave Amplitude

A

A in metres, the maximum magnitude of displacement

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8
Q

Wave Wavelength

A

In metres - the length of one whole wave cycle, from crest to crest or trough to trough

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9
Q

Wave Period

A

T in seconds, the time taken for a whole cycle to complete or to pass given point

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10
Q

Wave Frequency

A

f in hertz, the number of cycles per second

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11
Q

Wave phase

A

A measurement of the position of a certain point along the wave cycle

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12
Q

Wave phase difference

A

The amount one wave lags behind another

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13
Q

Phase or Phase difference are measured as [2]

A
  • Angles
  • Fractions of a cycle
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14
Q

Wave reflection def

A

The wave is bounced back when it hits a boundary

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15
Q

Wave refraction def

A

The wave changes direction as it enters a different medium

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16
Q

Frequency formula

A

f = 1/T
Where T = time period

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17
Q

Wave speed formulae

A

Wave speed = Distance travelled / Time taken
c = d/t

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18
Q

Speed, Wavelength, Ferquency

A

C = wavelength*freq

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19
Q

Speed of light in a vacuum

A

3*10^8

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20
Q

Transverse Waves def

A

Waves that oscillate at right angles to the direction of energy transfer

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21
Q

Examples of transverse waves [3]

A
  • All EM waves
  • Ripples
  • Waves on a string
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22
Q

How can waves be drawn [2]

A

Displacement against distance
Displacement against time

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23
Q

Longitudinal waves def

A

A wave in which the direction of oscillation in parallel to the direction of energy transfer

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24
Q

A longitudinal wave consists of ___&___

A

Compressions and rarefactions

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25
Q

Examples of longitudinal waves

A

Sound waves, shock waves

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26
Q

Polarised Wave def

A

A wave that only oscillates in one direction

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27
Q

Polarisation can only happen for ______

A

Transverse Waves

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28
Q

How do polarising filters work [4]

A
  • Light waves are a mixture of different directions of oscillation
  • Waves can be polarised when passed through a polarising filter
  • This causes them to oscillate in one direction only
  • Two polarising filters at right angles will cause no light to pass through
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29
Q

How can light be partially polarised

A

When light is reflected off some surfaces, it can become partially polarised

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30
Q

How do Polaroid sunglasses work

A

If you view reflected partially polarised light through a polarising filter at the correct angle, you can block out unwanted glare

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31
Q

Polarisation of TV and radio signals

A
  • TV and radio signals are polarised
  • The rods of the transmitting and receiving Ariel must be aligned to receive a strong signal
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32
Q

The principle of superposition

A

When two or more waves cross, the resultant displacement equals the vector sum of the individual displacements

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33
Q

Interference can be ___ or ____

A

Constructive or destructive

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34
Q

Constructive interference

A

Two crests result in a bigger crest

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35
Q

Destructive interference

A

A crest and a trough of equal magnitude result in nothing since the two displacements cancel each other out

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36
Q

If two points are in phase, they interfere _____

A

Constructively

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37
Q

When are two points exactly in phase

A

When they are both at the same point in the wave cycle, have the same displacement and velocity
Phase difference of a multiple of 360 degrees

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38
Q

When are points exactly out of phase

A

When they have a phase difference of an odd multiple of 180 degrees

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39
Q

To get an interference pattern, the two sources ____ __ ________

A

must be coherent

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40
Q

What makes two sources coherent

A

Two sources are coherent if they have the same wavelength and frequency, thus a fixed phase difference between them

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41
Q

Constructive or destructive interference depends on __________

A

The path difference

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42
Q

Constructive interference occurs when

A

Path difference = n*wavelength

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43
Q

Destructive interference occurs when

A

Path difference = (n+0.5)Wavelengths

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44
Q

Stationary wave def

A

A stationary wave is the superposition of two progressive waves with the same frequency and wavelength, moving in opposite directions

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45
Q

Key difference between progressive waves and stationary waves

A

Progressive waves transfer energy, stationary waves do not

46
Q

demonstrating stationary waves with a string

A

Set up a driving oscillator at one end of a stretched string with the other end fixed

47
Q

Wave Node Def

A

Where the amplitude of the oscillation is zero

48
Q

Wave antinode def

A

Points of maximum amplitude

49
Q

Resonant frequency

A

At resonant frequencies, an exact number of half wavelength fit on the string

50
Q

First harmonic

A

1/2 wavelengths

51
Q

Second Harmonic

A

1 wavelength

52
Q

Third Harmonic

A

3/2 wavelengths

53
Q

Demonstrating stationary waves with microwaves

A

INSERT IMAGE

54
Q

Demonstrating stationary waves in a column of air

A

INSERT IMAGE

55
Q

Investigating factors affecting the resonant frequencies of a string

A
56
Q

Factors affecting resonant frequencies on a string

A

Length
Weight
Tension

57
Q

Frequency of fist harmonic

A

INSERT PIC

58
Q

Diffraction def

A

The way that waves spread out as they come through a narrow gap or go round obstacles

59
Q

What does the amount of diffraction depend on

A

The wavelength of the wave compared to the size of the gap

60
Q

Diffraction if the gap is a lot bigger than the wavelength

A

Diffraction is unnoticeable

61
Q

Diffraction - If the gap is several wavelengths wide

A

Noticeable diffraction

62
Q

Diffraction - the gap is the same size as the wavelength

A

Most diffraction

63
Q

Diffraction - Gap is smaller than the wavelength

A

Waves are mostly reflected back

64
Q

Conditions to observe a clear diffraction pattern

A

A monochromatic, coherent light source

65
Q

Demonstrating light diffraction patterns with a laser

A

INSERT PIC

66
Q

How can light diffraction be demonstrated using a laser

A

Passing a wave through a narrow slit and projecting on a slit.
The wavelength is about the same size as the appeture

67
Q

Observation for light diffraction using a laser [2]

A

A central bight fringe (central maximum)
Dark and bright fringes alternating on either side

68
Q

How does the diffraction of white light create a spectra of colors [3]

A
  • White light is a mixture of wavelengths
  • When passed through the slit, all wavelengths are diffracted by a different amount
  • This produces a spectra of colors
69
Q

Light intensity [2]

A

The number of photons
Power per unit area

70
Q

Light intensity of the central maximum

A

Highest Light intensity at the centre of the central maximum

71
Q

Effect of increasing slit width on the width of the central maximum, light intensity [3]

A

Decreases amount of diffraction
Narrower central maximum
Higher light intensity

72
Q

Effect of increasing wavelength on the width of the central maximum, light intensity [3]

A

Increases amount of diffraction
Wider Central Maximum
Lower light intensity

73
Q

Demonstrating two source interference in water and sound

A

Two coherent sources with the same wavelength and frequency

74
Q

Conditions for Youngs Double slit experiment

A

Two coherent monochromatic sources, the slit size is about the same as the wavelength

75
Q

Youngs double slit setup

A

INSERT IMAGE

76
Q

Laser Safety [5]

A

1) Never shine the laser towards a person
2) Wear laser safety goggles
3) Avoid shining the laser beam at a reflective surface
4) Have a warning sign on display
5) Turn the laser off when it is not needed

77
Q

Microwaves two source interference setup

A

IMAGE

78
Q

Youngs double slit fringe spacing formula

A

Fringe Spacing = (Wavelength*Distance)/slit spacing

79
Q

How is the Youngs Experiment evidence for the Wave nature of EM radiation [2]

A
  • Diffraction and interference are both uniquely wave properties
  • The experiment shows that light can diffract and interfere
80
Q

Interference patterns get ______ when you diffract through more slits

A

Sharper, the multiple beams reinforce the pattern

81
Q

Why are sharper beams more useful

A

More accurate measurements

82
Q

Observations for a monochromatic diffraction grating

A

1) All the Maxima are sharp lines
2) There is a line of maximum brightness at the centre called the zero order line
3) The following Paris are called the first order lines and so on

83
Q

Diffraction grating formula

A

slit spacing* sin () = n wavelength

84
Q

Diagram of diffraction gratings for monochromatic light

A

INSERT PIC

85
Q

Deriving the equation for diffraction gratings

A
  • triangle, d sin a = wavelength
86
Q

Diffraction gratings, bigger wavelength

A

Sin () is bigger
so () is bigger
The pattern is more spread out

87
Q

Diffraction gratings, bigger slit spacing

A

Sin () is smaller
() is smaller
The pattern is less spread out

88
Q

Diffraction gratings for values of sin() >1

A

Do not exist since sin() can not be greater than 1

89
Q

What happens when diffraction gratings are used for non-monochromatic light

A
  • White light is made of a mix of wavelengths
  • Different wavelengths diffract different amounts
  • Each order in the pattern becomes a spectrum
  • Zero order stays white
90
Q

Image, diffraction of white light through a grating

A

INSERT IMAGE

91
Q

Uses of diffraction gratings [4]

A
  • More accurate
  • Called X ray crystallography
  • Used to measure the spacing between atoms
  • Used to discover DNA
92
Q

Refractive index def

A

A measure of how much light slows down in a material / how much light will defreact

93
Q

Why diffraction happens

A

Light slows down in materials because it interacts with the particles in it. The more dense the material is, the more light slows down. The slowing down causes light to bend at a boundary

94
Q

Formaula for absolute refractive index

A

n = Speed of light in a vacuum / speed of light in medium

95
Q

Formula for relative refractive index

A

1n2 = c1 / c2

96
Q

relative refractive index in terms of absolute refractive index

A

1n2 = n2 / n1

97
Q

Absolute refractive index vs relative refractive index

A

the absolute refractive index is the property of a single material only while the relative refractive index is the property of the interface between two materials

98
Q

Refractive index of air / vaccum

A

1

99
Q

When light enters a denser medium

A

Refraction towards the normal

100
Q

When light enters a less dense medium

A

Refraction away from the normal

101
Q

Snells Law

A

n1 * sin()1 = n2 * sin()2

102
Q

Critical Angle def

A

The maximum angle unto which refraction occurs. At () greater than the critical angle, all the light is reflected back into the material, total internal reflection

103
Q

Critical angle formula

A

1n2 = sin ()c

104
Q

How do optical fibres work [3]

A

1) Optical fibres are made of a material with a high refractive index which is surrounded by a material with a low refractive index
2) This means that light is totally internally reflected
3) light bounces from side to side at angles greater than the CIR, moving along the firbre

105
Q

Why is the cladding needed on an optical fibre

A

Protects fibres from scratches which would cause light to escape
Allows TIR to happen

106
Q

Absorption in optical fibres

A

Causes loss in amplitude
Energy is absorbed by the material

107
Q

Modal Dispersion in optical fibres

A

Light rays enter at different angles and take different paths, some take a longer path

108
Q

Preventing modal dispersion in optical fibres

A

Single mode fibre that only lets light take one path, preventing modal dispersion

109
Q

Material dispersion in optical fibres

A

Light consists of different wavelengths so some wavelengths reach the end of the fibre faster than others

110
Q

Preventing material dispersion in optical firbres

A

Using monochromatic light

111
Q

Result of dispersion in optical fibres

A

INSERT IMAGE
pulse broadens at each end

Physics A level (12 decks)

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