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A-Level Physics > 3. Waves > Flashcards

3. Waves Flashcards

1
Q

What is a progressive wave?

A

a wave that transfers energy from one place to another without transferring material

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2
Q

What causes a wave?

A

something that makes particles or fields oscillate

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3
Q

What three things can happen to a wave?

A

reflection, refraction, diffraction

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4
Q

Define displacement

A

the distance and direction of a particle on a wave from its position of equilibrium

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5
Q

Define amplitude

A

the maximum magnitude of displacement from the position of equilibrium

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6
Q

Define period

A

the time taken for one complete cycle to pass a given point

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7
Q

Define frequency

A

the number of complete wave cycles per second passing a given point

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8
Q

Define wavelength

A

the length of one whole oscillation (the distance between successive peaks/troughs)

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9
Q

Define speed

A

the distance travelled by the wave per unit time

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10
Q

Define phase

A

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

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11
Q

Define phase difference

A

the amount by which one wave lags behind another wave

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12
Q

What are the units of phase and phase difference?

A

radians, degrees or fractions of a cycle

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13
Q

What is a transverse wave?

A

the oscillations of particles are perpendicular to the direction of energy transfer

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14
Q

What is a longitudinal wave?

A

the oscillations of particles are parallel to the direction of energy transfer

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15
Q

State three types of transverse waves

A
  • electromagnetic (EM) waves
  • water waves
  • earthquake S-waves
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16
Q

State two types of longitudinal waves

A
  • sound waves

- earthquake P-waves

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17
Q

Give one characteristic of longitudinal waves

A

areas of compression and rarefaction

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18
Q

At what speed do EM waves travel in a vacuum?

A

3 x 10^8 m/s

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19
Q

What does it mean if a wave is unpolarised?

A

the particles/field oscillate in more than one plane

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20
Q

What does it mean if a wave is polarised?

A

the particles/field oscillate in just one plane

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21
Q

What can be used to polarise unpolarised light?

A

a polarising filter

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22
Q

What does a polarising filter consist of?

A

very small openings which can be orientated horizontally or vertically

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23
Q

How can all light be blocked?

A

using a combination of vertical and horizontal filters

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24
Q

Are the signals sent from TV transmitters polarised or unpolarised?

A

polarised

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25
Q

Why must TV aerials be aligned correctly?

A

to receive a signal, the aerial must be aligned in the same plane of polarisation as the polarised wave

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26
Q

How do Polaroid sunglasses reduce glare?

A
  • the reflected light is partially polarised
  • Polaroid sunglasses have polarising filters which, when perpendicular to the plane of polarisation, can block out some of the polarised light
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27
Q

State three examples of surfaces which act as natural polarising filters

A
  • water
  • glass
  • tarmac
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28
Q

How can some surfaces act as natural polarising filters?

A

when these surfaces are hit with unpolarised light, the light that reflects off is partially polarised, making it easier to block with a filter

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29
Q

What determines how much light is polarised by some surfaces?

A

the angle of the surface

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30
Q

State the equation linking wave speed, wavelength and time period

A

wave speed = (wavelength)/(time period)

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31
Q

How are frequency and period related?

A

frequency = 1/period

inversely proportional

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32
Q

State the equation linking wave speed, frequency and wavelength

A

wave speed = frequency x wavelength

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33
Q

What is superposition used to describe?

A

two or more waves combining together

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34
Q

What is the principle of superposition of waves?

A

when two waves meet, the total displacement at a point is equal to the sum of each individual displacement of each wave at that point

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35
Q

What can the superposition of two or more waves result in?

A

interference

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36
Q

What are the three main types of interference?

A
  • constructive interference
  • destructive interference
  • total destructive interference
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37
Q

What is constructive interference?

A

when two waves meet, if their displacements are in the same direction, the displacements combine to give a bigger displacement

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38
Q

What is destructive interference?

A

if a wave with a positive displacement meets and wave with a negative displacement, they partially cancel each other out

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39
Q

What is total destructive interference?

A

if two waves with equal and opposite displacements (the same amplitude) meet, they cancel each other out completely

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40
Q

When are two points on a wave in phase?

A

if they are both at the same point in the wave cycle e.g. 0, 360, 720…degrees

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41
Q

What must two waves have to be in phase?

A

the same frequency and the same wavelength

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42
Q

What is a stationary wave?

A

the superposition of two progressive waves travelling in opposite directions, with the same frequency/wavelength and similar amplitude

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43
Q

How does a stationary wave differ from a progressive wave?

A

no energy is transferred

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44
Q

What happens when the two progressive waves forming a stationary wave overlap?

A

a larger stationary wave is created via constructive interference

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45
Q

What happens when the two progressive waves forming a stationary wave line up with peak to trough?

A

the stationary wave becomes flat via total destructive interference

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46
Q

What are the two points that will form on a stationary wave?

A

nodes and anti-nodes

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47
Q

What are the characteristics of a node?

A
  • points of no displacement
  • constantly remain stationary
  • total destructive interference is always occurring here
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48
Q

What are the characteristics of an anti-node?

A
  • points of maximum displacement
  • where amplitude reaches a maximum
  • constructive interference is occurring here
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49
Q

What is the distance between two nodes or between two anti-nodes?

A

half a wavelength

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50
Q

What is the easiest way to make a stationary wave?

A

reflect a wave back on itself

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51
Q

What is the difference between the phase difference of a progressive wave and a stationary wave?

A
  • phase difference can be any value between two points on a progressive wave
  • two points on a stationary wave can only be in phase or in anti-phase
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52
Q

When can stationary waves form?

A

when a whole number of half wavelengths can fit

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53
Q

What are resonant frequencies/harmonics?

A

when an exact number of half wavelengths fit on the string

54
Q

What is the lowest possible resonant frequency called?

A

the first harmonic/the fundamental frequency

55
Q

In what three ways can the fundamental frequency of a string be changed?

A
  • changing the length of the string
  • changing the tension in the string
  • changing the mass per unit length of the string
56
Q

What happens to the fundamental frequency as wavelength decreases?

A

the fundamental frequency will be higher

57
Q

How can you change the tension in the string?

A

changing the amount of mass hanging from the string

58
Q

What happens to the speed of the waves when tension is increased?

A

speed increases

59
Q

What happens to the fundamental frequency when the tension in the string is lower?

A

low tension = slower waves = lower frequency

60
Q

What happens to the speed of waves when the string is heavier?

A

the waves will move slower

61
Q

What happens to the fundamental frequency when the mass of the string increases?

A

heavier string = slower waves = lower frequency

62
Q

How much diffraction do you get when the gap is significantly larger than the wavelength?

A

no diffraction

63
Q

How much diffraction do you get when the gap is a bit larger than the wavelength?

A

some diffraction

64
Q

How much diffraction do you get when the gap is the same size as the wavelength?

A

maximum diffraction

65
Q

What happens when the gap is smaller than the wavelength?

A

the wave can’t pass through

66
Q

What do you need to use to get light to diffract through a single slit?

A

coherent, monochromatic light

67
Q

What is the meaning of the term monochromatic light?

A

light of a single wavelength

68
Q

What sort of waves diffract?

A

all waves

69
Q

What does the term coherent mean?

A

light has the same frequency and a fixed phase difference

70
Q

What is the easiest way to achieve coherent, monochromatic light?

A

using a laser

71
Q

What is a fringe pattern?

A

the diffraction pattern that is seen on the screen

72
Q

What is a fringe pattern made up of?

A

a bright central fringe (central maximum) with alternating dark and bright fringes on either side

73
Q

What causes bright fringes?

A

constructive interference, where waves from across the width of the slit arrive at the screen in phase

74
Q

What causes dark fringes?

A

total destructive interference, where waves from across the width of the slit arrive at the screen in anti-phase

75
Q

Where is the brightest part of the pattern? Why? What is this known as?

A
  • the middle, where the light is most intense

- central maximum

76
Q

What is intensity?

A

the power per unit area

77
Q

How does the width of the central maximum compare to the other maxima?

A

the central maximum is twice the width of the other maxima

78
Q

What is two source interference?

A

when the waves from two sources overlap with each other

79
Q

How can two source interference be achieved?

A
  • using two speakers producing sound waves
  • two pebbles dropped into a pond
  • a laser beam shone through two small slits
80
Q

What do the waves need to be in order to achieve two source interference?

A

coherent

81
Q

Where do you get a point of maximum displacement? What is this known as?

A
  • where a peak/trough meets a peak/trough (where constructive interference takes place)
  • a maxima
82
Q

Where do you get a point of no displacement? What is this known as?

A
  • where a peak meets a trough (where total destructive interference takes place)
  • a minima
83
Q

What would two source interference using sound waves result in?

A

loud and quiet spots

84
Q

What is produced when laser light is shone through two small slits?

A

a fringe (interference pattern)

85
Q

How does the pattern produced by two small slits differ from a single slit pattern?

A
  • the pattern is much more intense and clearer

- all of the maxima are exactly the same width

86
Q

What are maxima?

A

point of constructive interference

87
Q

What are minima?

A

points of total destructive interference

88
Q

What is path difference?

A

a measure of how much one wave has travelled further than another

89
Q

What happens when the path difference is zero?

A
  • waves arrive in phase
  • constructive interference occurs
  • bright spot forms
90
Q

What happens when the path difference is half a wavelength?

A
  • waves arrive in anti-phase
  • total destructive interference occurs
  • dark spot forms
91
Q

What happens when the path difference is one wavelength?

A
  • waves arrive in phase
  • constructive interference occurs
  • bright spot forms
92
Q

What safety precautions must be made when using a laser?

A
  • never shine towards a person
  • wear laser safety goggles
  • avoid shining at a reflective surface
  • having a warning sign
  • turn the laser off when not needed
93
Q

What is produced when a laser is shone through one slit?

A
  • a very blurry interference pattern

- central maximum twice the size

94
Q

What is produced when a laser is shone through two slits?

A

a more intense pattern of maxima and minima due to interference

95
Q

What is produced when a laser is shone through a diffraction grating?

A
  • light diffracts through multiple slits

- produces a very intense pattern

96
Q

What does a diffraction grating contain?

A

lots of equally sized slits very close together (hundreds of slits per millimetre)

97
Q

What are orders?

A

the bright spots formed by a diffraction pattern using a diffraction grating

98
Q

What is the straight through position of the diffracted light called?

A

the zero order

99
Q

What are the two either side of the zero order called?

A

the first order

100
Q

How do you calculate the maximum number of orders?

A

(distance between slits)/wavelength

- rounded down

101
Q

What happens when you diffract white light?

A
  • central fringe will be white
  • subsidiary maxima will each be composed of an entire spectrum
  • red will be furthest from the centre and blue/violet will be closest
  • there will be less fringe spacing with smaller dark fringes and wider maxima
102
Q

Where does light travel fastest?

A

in a vacuum

103
Q

What is diffraction?

A

when a wave bends as it passes through a gap or around an obstacle

104
Q

What is refraction?

A

when a wave changes speed and direction as it enters a different medium

105
Q

What is reflection?

A

when a wave is bounced back when it hits a boundary

106
Q

What is the absolute refractive index of a material?

A

the ratio between the speed of light in a vacuum and the speed of light in that material

107
Q

What is the refractive of index of air?

A

1

108
Q

What happens when a wave passes into a denser material?

A
  • the wave slows down
  • the wavelength decreases
  • the frequency stays the same
109
Q

When a wave travels into a less dense material, does it bend towards or away from the normal? Why?

A
  • bends away from the material

- the wave speeds up

110
Q

What is the angle of incidence?

A

the angle that incoming light makes to the normal

111
Q

What is the angle of refraction?

A

the angle the refracted ray makes with the normal

112
Q

What happens to a small amount of light when travelling from a high to a low refractive index?

A

it gets reflected

113
Q

What is the greatest value of the angle of refraction?

A

90 degrees

114
Q

What is the critical angle?

A

the value of the angle of the incidence when the angle of refraction reaches 90 degrees and the light cannot escape material 1

115
Q

What are the two criteria needed to achieve the critical angle?

A
  • can only be achieved when going from a higher refractive index to a lower refractive index
  • when the angle of refraction is 90 degrees
116
Q

How can total internal reflection (TIR) be achieved?

A
  • if the angle of incidence is higher than the critical angle
  • when light is going from a higher to a lower refractive index
117
Q

What is a fibre optic cable?

A

a very thin flexible tube of glass or plastic fibre that can carry light signals over long distances and round corners using total internal reflection

118
Q

Does the core of a fibre optic cable have a low or a high refractive index?

A

the core has a very high refractive index

119
Q

Does the cladding of a fibre optic cable have a low or a high refractive index?

A

the cladding has a very low refractive index

120
Q

What are the advantages of using fibre optic cables to transmit data rather than copper cables?

A
  • signal can carry more information as it has a higher frequency
  • light doesn’t heat up fibre so no energy lost as heat
  • no electrical interference
  • much cheaper to produce
  • signal can travel a long way quickly without much signal loss
121
Q

State two functions of the cladding of fibre optic cables

A
  • to protect the core from scratches

- to prevent crossover of signal/to ensure security of data/prevent loss of information

122
Q

What are the three main problems with fibre optic cables?

A
  • absorption
  • modal dispersion
  • material dispersion
123
Q

How does absorption cause signal degradation?

A
  • some of the signal’s energy is absorbed by the material due to imperfections in the fibre e.g. dust and bubbles
  • this energy loss results in the amplitude of the signal being reduced
124
Q

What do both modal and material dispersion cause?

A

pulse broadening

125
Q

What is the effect of pulse broadening?

A
  • the received signal is broader than the initial signal

- broadened pulses can overlap each other, leading to information loss

126
Q

How does modal dispersion occur?

A

light rays enter the optical fibre at different angles, causing them to take different paths down the fibre

127
Q

How can modal dispersion be reduced?

A

by using a single-mode fibre

128
Q

What does a single-mode fibre do?

A

only allows light to follow a very narrow path

129
Q

How does material dispersion occur?

A
  • different wavelengths are refracted by different amounts
  • different wavelengths slow down by different amounts in a material
  • so different parts of the signal will arrive at different times
130
Q

How can material dispersion be reduced?

A

by using monochromatic light

131
Q

What can be used to help reduce the signal degradation from both absorption and dispersion?

A

optical fibre repeaters

132
Q

What do optical fibre repeaters do?

A

they can be used to regenerate the signal every so often

A-Level Physics (15 decks)

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