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Waves Flashcards

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1
Q

What is a wave?

A

A regular disturbance that carries energy from one place to another.

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2
Q

What does a wave transport?

A

Energy, not matter

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3
Q

When a wave is present in a medium, what happens to the individual particles?

A

They are temporarily displaced from their rest position; there is always a force acting upon the particles that restores them to their original position.

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4
Q

What are the two ways of showing wave motion in a graph?

A
  • displacement-time
  • displacement-distance
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5
Q

What is displacement?

A

Instantaneous distance from the equilibrium level

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6
Q

What is amplitude?

A

The maximum displacement from the equilibrium position

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7
Q

What is wavelength?

A

The distance between any two points on adjacent cycles which are vibrating in phase.

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8
Q

What is the meaning of ‘in phase’?

A

At the same point in the cycle

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9
Q

What is time period?

A

The time taken for one complete oscillation

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10
Q

What is frequency?

A

The number of oscillations in one second

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11
Q

What do the symbols stand for? T = 1/f

A

T = time period, f = frequency

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12
Q

What do the symbols stand for? c = fλ

A

c = wave speed, f = frequency, λ = wavelength

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13
Q

Derivation of the wave equation?

A

Speed = distance / time
Speed = wavelength / period

Speed = wavelength / (1/frequency)
Speed = wavelength x frequency

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14
Q

What is phase difference?

A

The difference between two waves having the same frequency and referenced to the same point in time

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15
Q

What is phase difference expressed in?

A

Degrees, radians or fractions of a cycle

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16
Q

Would two oscillators with the same frequency and different phases have a phase difference?

A

Yes - they would be out of phase with each other

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17
Q

What is the range of values for phase difference?

A
  • Degrees - 0 to 360
  • Radians - 0 to 2π
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18
Q

What is antiphase?

A

When the phase difference is 180 degrees (π radians)

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19
Q

What is the equation for phase difference?

A

x/λ x 360 (degrees)

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20
Q

What are transverse waves?

A

When the displacement is at right angles to the direction of energy propagation

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21
Q

What are longitudinal waves?

A

When the displacement is parallel to the direction of energy propagation

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22
Q

What type of wave is light?

A

Transverse, electromagnetic

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23
Q

What type of wave is sound?

A

Longitudinal, mechanical

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24
Q

What are mechanical waves?

A

Waves that travel by vibrating particles in a medium

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25
Q

Can mechanical waves travel in a vacuum?

A

No

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26
Q

What are electromagnetic waves?

A

Waves that can travel through a vacuum

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27
Q

What is speed of light in a vacuum?

A

3 x 10⁸ m/s

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28
Q

What happens when an electromagnetic wave hits a surface?

A

The wave can be reflected, transmitted or absorbed

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29
Q

What happens to an object when it absorbs an electromagnetic wave?

A

Its temperature increases

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30
Q

In which planes can the displacements of oscillations in transverse waves be?

A

In all planes

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31
Q

What are plane-polarised waves?

A

Have the oscillations in one plane only

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32
Q

How is light polarised?

A
  • by absorbing all planes of oscillation except one
  • by reflection
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33
Q

What is Polaroid plastic formed from?

A

Many tiny crystals, all lined up

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34
Q

Which planes of oscillations does Polaroid plastic absorb?

A

All of them except the vertical one

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35
Q

When light is polarised by reflection, which way is the plane of polarisation?

A

Horizontal

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36
Q

What happens if you wear sunglasses made from Polaroid plastic and look at water?

A

Reflection off the water surface is absorbed because its plane of polarisation is perpendicular to that of the Polaroid

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37
Q

What happens if two pieces of Polaroid are ‘crossed so that their transmission planes are at right angles?

A

No light will get through

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38
Q

How can transverse and longitudinal waves be distinguished?

A

Transverse waves can be polarised; longitudinal cannot

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39
Q

What are electromagnetic waves a combination of?

A

Electric and magnetic field waves produced by moving charges

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40
Q

What is polarisation used in?

A
  • sunglasses
  • alignment of aerials for transmission and reception
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41
Q

When are superposed?

A

When two waves of the same type are in the same place at the same time

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42
Q

How is the resultant displacement at any point found when two waves are superposed?

A

By adding displacements of each separate wave

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43
Q

What is interference?

A

The adding together of waves

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44
Q

What is the principle of superposition?

A

At a point where two or more waves meet, the instantaneous displacement (amplitude) is the vector sum of the individual displacements due to each wave at that point

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45
Q

When will interference be constructive?

A

When waves are in phase and the same frequency

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46
Q

What must the path difference be for constructive interference?

A

nλ (where n is a whole number of wavelengths)

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47
Q

When will interference be destructive?

A

When waves are in antiphase and have the same frequency

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48
Q

What must the path difference be for destructive interference?

A

(1 + n/2)λ i.e. an odd number of wavelengths

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49
Q

What does it mean when waves are phase linked?

A

The waves have a constant phase difference

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50
Q

When are superposed waves easier to ‘see’?

A
  • the waves are of similar amplitude (increased constant between maxima and minima)
  • the waves have similar frequencies (otherwise the interference patterns change so fast that they are difficult to detect)
  • the waves have a constant phase difference
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51
Q

What are some examples of coherent sources?

A
  • Light produced by a laser
  • Sound from two loudspeakers connected in parallel
  • Light emerging from two apertures illuminated by the same source
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52
Q

What are coherent sources?

A

Sources that have synchronised phase changes, as well as the same frequency and wavelength

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53
Q

What are nodes?

A

On stationary waves, points that are always at equilibrium and zero oscillation

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54
Q

What are antinodes?

A

On stationary waves, points of maximum oscillation

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55
Q

On a stationary wave, what is the distance from one node to the next?

A

1/2 λ

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56
Q

How are stationary waves formed on a string?

A
  • Vibrator moves up and down, sending travelling wave down the cord
  • Wave is reflected at the end, so the two travelling waves overlap and interfere
  • Has antinodes and nodes; distance between nodes = 1/2λ
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57
Q

Comparison of the frequencies of particles in stationary and travelling waves?

A

Stationary - all particles (except nodes) have the same frequency

Travelling - all particles have the same frequency

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58
Q

Comparison of the amplitudes of particles in stationary and travelling waves?

A

Stationary - varies from 0 (nodes) to maximum (antinodes)

Travelling - same for all particles

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59
Q

When does resonance occur?

A

When the frequency driving the system matches the natural frequency of the system

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60
Q

Comparison of the phase difference between two particles in stationary and travelling waves?

A

In stationary waves, points between two adjacent nodes or separated by an even number of nodes are in phase, whereas points separated by an odd number of nodes are in antiphase. (mπ where m is the number of nodes between the two particles).

In travelling waves phase difference is (2πx/λ where x is the distance apart)

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61
Q

Comparison of the energy of particles in stationary and travelling waves?

A

Stationary - energy stored and not transferred

Progressive - energy transferred

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62
Q

What is a stationary wave?

A

Where energy is stored rather than transmitted - formed when two coherent waves travelling in opposite directions interfere to produce nodes and antinodes

63
Q

What can increase the pitch of a note on a guitar string?

A
  • Increase the tension
  • Decrease the length of the string
  • Decrease the thickness of the string
64
Q

What does the first harmonic depend on?

A

Tension T, length l, and its mass per unit length

65
Q

What do the symbols stand for? f = (1/2L) x √(T/μ)

A

f = frequency, l = length, T = tension, μ = mass per unit length

66
Q

What happens when the air at one end of the tube/pipe is caused to vibrate?

A

A longitudinal wave travels down the tube and is reflected at the opposite end, forming a stationary wave.

67
Q

Where are the anti-nodes in an open pipe?

A

At both ends

68
Q

Why are waves reflected at the ends of open pipes?

A

Air acts as a barrier outside

69
Q

At resonant frequencies in a closed pipe, where are the nodes and anti-nodes?

A

Node at closed end, anti-node at open end

70
Q

Describe the amplitude of the particles in a closed pipe.

A

Amplitude decreases gradually from the maximum at the open end to zero at the closed end

71
Q

Which harmonics can be obtained in an open pipe?

A

All of them

72
Q

Which harmonics can be obtained in a closed pipe?

A

Odd harmonics

73
Q

Why are standing waves only produced at certain frequencies?

A

There needs to be a whole number of stationary wave loops fitting into the length of the string

74
Q

What does the double slit interference pattern consist of?

A

Equidistant parallel fringes alternating between: maxima (constructive interference) and minima (destructive interference).

75
Q

What happens when waves are travelling in the same direction and overlap?

A

They interfere

76
Q

What does it mean if two sources are coherent?

A

They emit identical waves which start in phase

77
Q

How can light that is in phase be produced for the double slit experiment?

A
  • use two coherent sources
  • use a single source with double slits
78
Q

What does it mean if two sources are coherent?

A

They have the same frequency, wavelength and synchronised phase changes

79
Q

What do the symbols stand for?

w = λD / s

A

w = fringe spacing, λ = wavelength, D = slit to screen distance, s = spacing between slits

80
Q

For small angles, what does sinθ equal?

A

θ

81
Q

If all types of wave interfere, why can’t we see interference patterns?

A

To obtain a clear interference pattern, it requires two coherent waves of monochromatic light. Light is usually emitted in bursts of waves, after which is a random phase change.

82
Q

How is light usually emitted?

A

In bursts of waves, each burst lasting 10⁻⁹ s, after which there’s a random phase change.

83
Q

What is a monochromatic source?

A

A source of a single wavelength

84
Q

Is there interference when two separate light sources are used?

A

Never

85
Q

What is fringe separation?

A

The distance between neighbouring bright fringes

86
Q

What happens to the double slit interference pattern if green light is used instead of red?

A

Wavelength is decreased so distance between adjacent fringes decreases

87
Q

What happens to the double slit interference pattern if white light is used?

A

The central fringe is white with red edges. Other fringes will be spectra with the blue end towards the middle of the overlap area.

88
Q

What happens to the double slit interference pattern if the screen is moved further awat?

A

D increases so the distance between adjacent fringes increases

89
Q

What happens to the double slit interference pattern if the phase difference between two sources is changed to 180 degrees?

A

The maxima will become minima and vice versa

90
Q

What happens to the double slit interference pattern if both slits are made narrower?

A

Wider interference so there are more dots, but fainter as there is less light through (x increases)

91
Q

What happens to the double slit interference pattern if one slit is narrower than the other?

A

The waves don’t fully cancel out

92
Q

How can you increase x in the Young’s slit experiment?

A

Increasing D - measurement is easier and more accurate but fringe intensity decreases

Decreasing a - practical limit to this

Increasing wavelength

93
Q

Can mechanical waves interfere?

A

Yes

94
Q

What is diffraction?

A

When a wave passes through a gap and spreads out

95
Q

What happens to diffraction when the gap width decreases?

A

Diffraction increases

96
Q

When is diffraction strongest?

A

When the gap width is similar to the wavelength of the wave

97
Q

Why do waves passing through a single gap interfere?

A

Only one slit but more than one wave. The single slit can be though of as a large number of sources next to each other. Each ‘source’ produces a coherent wave, which overlap and interfere.

98
Q

What happens, when light is shone on a diffraction grating, when the wavelength is increased?

A

Short λ (like blue light) has a narrow diffraction pattern

Long λ (like red light) has a broad diffraction pattern

99
Q

What happens when white light is shone on the diffraction grating instead of monochromatic?

A

White light yields less clear patterns (as position of dark bands depends on λ)

Colours appear; only central band is white

100
Q

What is the difference between a single and double slit pattern?

A

Single slit - central maximum is twice the width of the other fringes

Double slit pattern has equally spaced fringes

101
Q

How many slits are on a diffraction grating?

A

1000s

102
Q

Which method produces a better diffraction pattern?

A

Diffraction grating - as not much light gets through the doubles slits so are dim and unclear

103
Q

What is a diffraction grating?

A

A set of slits for light waves to pass through

104
Q

How do you calculate the number of slits per meter on a diffraction grating?

A

x = 1/d

105
Q

What do the symbols stand for?

nλ = dsinθ

A

n = order of maxima, λ = wavelength, d = slit separation, θ = angle

106
Q

In the diffraction grating equation, what is the significance of sinθ never being greater than one?

A

There is a limit to the number of spectra that can be obtained

107
Q

What is the zero-order maximum?

A

The waves that produce the bright spot straight on. Paths are all the same length, so phase difference is zero

108
Q

What is the equation for the maximum number of orders?

A

n = d/λ

109
Q

Why is sinθ not present in the equation for the maximum number of orders?

A

It will give a maximum when sinθ = 1 so cancels out

110
Q

When using the equation n = d/λ, which quantity must be a whole number?

A

n

111
Q

Which is more accurate, the diffraction grating or the double slit method?

A

Diffraction grating

112
Q

Why is the diffraction grating more accurate than the double slits?

A

Double slits - fringes formed are slight blurred so there are large errors

Diffraction grating - images are clear and measurements accurate. Final result is an average of several calculations

113
Q

What can diffraction gratings be used for?

A

Analysis of spectra

114
Q

What is an optical fibre?

A

A long, thin, cylindrical core of glass, encased in a cladding of glass of lower refractive index

115
Q

What is refraction?

A

A change in the direction of light as it passes across a boundary between two transparent substances

116
Q

What happens in terms of refraction, if light passes across a boundary at 90 degrees to a surface?

A

It doesn’t refract

117
Q

What is a refractive index?

A

A measure of the optical density of a material relative to air

118
Q

What is the approximate refractive index of air?

A

1

119
Q

What does the symbols stand for?

n=c/cs

A

n = refractive index, c = speed of light in a vacuum, cs = speed of light in the medium

120
Q

What is the definition of Snell’s law?

A

The ratio of the sines of the angles of incidence and refraction are constant when it passes between two given media

121
Q

What do the symbols stand for?

n₁sinθ₁ = n₂sinθ₂

A

n = refractive index, θ = angle

122
Q

What are the two conditions for total internal reflection?

A
  • Light passes from a more optically dense medium to a less optically dense medium
  • Angle of incidence > critical angle
123
Q

What is total internal reflection?

A

When light passes from a more to less dense medium and the angle of incidence is greater than the critical angle, all light is reflected back to the less dense medium

124
Q

What is the critical angle?

A

The angle of incidence to which the angle of refraction is 90 degrees

125
Q

What is the angle of refraction when the angle of incidence is equal to the critical angle?

A

90°

126
Q

What do the symbols stand for?

sinθc = n₁ / n₂

A

θc = critical angle, n₁ = the first material, n₂ = the material it enters

127
Q

How does light travel along an optical fibre?

A

By total internal reflection, only escaping when it reaches the other end

128
Q

What is an endoscope?

A

A medical instrument that uses optical fibres to look inside the body

129
Q

What do endoscopes consist of?

A

A coherent bundle of fibres (lens system) and an incoherent bundle of fibres (light delivery system)

130
Q

What happens if a fibre is bent too tightly?

A

Angle of incidence will be less than the critical angle and light will escape

131
Q

What can endoscopes be used to look at?

A

Digestive, respiratory and female reproductive systems

132
Q

What are the positives of endoscopes?

A

Can diagnose patients without an incision, often without anesthetic

133
Q

In an optical fibre, when will total internal reflection occur?

A

As long as θ is larger than the critical angle

134
Q

In medicine, what are the uses of optical fibres?

A

Endoscopes, lasers - burn tissue to heal wound

135
Q

What is a coherent bundle of fibres?

A

Where the fibres stay in the same relative position along their length

136
Q

What are some of the problems for optical fibres?

A
  • Scratches can cause light to leak
  • Two fibres touching can cause light to pass from one to the other
  • Dispersion
137
Q

How can scratches and cross talk be resolved when using optical fibres?

A

Using cladding

138
Q

Does cladding have a lower or higher refractive index than the core?

A

Lower

139
Q

How is light sent down an optical fibre?

A

In pulses or bursts

140
Q

How can a pulse be distorted in an optical fibre?

A

Absorption - some energy is absorbed so the pulse has a lower amplitude

Dispersion - causes pulse broadening

141
Q

What are the two types of dispersion?

A

Modal - light takes different paths through the core

Material - different wavelengths of light travel at different speeds

142
Q

How does modal dispersion occur?

A

A pulse can take a variety of different paths through a fibre, meaning a single pulse can spread out over time

143
Q

How can modal dispersion be decreased?

A

Use a narrow core

144
Q

How does material dispersion occur?

A

Different wavelengths of light travel at different speeds through the core. A pulse will reach the end of the fibre at different times if there are multiple wavelengths, which is pulse broadening

145
Q

How can material dispersion be reduced?

A

Using monochromatic light

146
Q

What colour of light should be used in an optical fibre

A

Red - it travels faster

147
Q

What is it called when, in a prism, white light is split into a spectrum of colours?

A

Dispersion

148
Q

In a prism, what colour light is refracted more: red or blue?

A

Blue

149
Q

Why is blue light refracted more than red in a prism?

A

Blue light travels more slowly in glass than red light

150
Q

When is pulse distortion more of a problem?

A

When the pulses are very short and close together

151
Q

When are single fibres used?

A

In communications

152
Q

When are bundles of fibres used?

A

In endoscopes

153
Q

What is pitch?

A

Pitch is a term used to describe how high or low a note seems to be. The pitch of a note depends on the frequency

Physics (8 decks)

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