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AQA A Level Physics > 3. Waves > Flashcards

3. 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 (undisturbed) 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 cycle (oscillation/wave)

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10
Q

What is frequency?

A

The number of oscillations (or cycles) in one second

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11
Q

Equation for time period?

A

T = 1/f

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12
Q

What is the wave speed equation?

A

c =fλ

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13
Q

Derivation of the wave equation?

A
  • speed = distance / time = wavelength / period
  • c = λ/T = λ/f⁻¹
  • c = λf
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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)

* x/λ x 2π (radians)

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20
Q

What are transverse waves?

A

When the displacement is at right angles to the direction of the wave

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21
Q

What are longitudinal waves?

A

When the displacement is parallel to the direction of the wave

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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 the 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 what 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 oscillation 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

Why do you not get dazzled by the reflected glare from water when wearing Polaroid sunglasses?

A

The plane of polarisation of the water is perpendicular to that of the Polaroid

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38
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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39
Q

How can transverse and longitudinal waves be distinguished?

A

Transverse waves can be polarised; longitudinal cannot

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40
Q

What are electromagnetic waves a combination of?

A

Electric and magnetic field waves produced by moving charges

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41
Q

What is polarisation used in?

A
  • sunglasses

* alignment of aerials for transmission and reception

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42
Q

When are waves superposed?

A

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

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43
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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44
Q

What is interference?

A

The adding together of waves

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45
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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46
Q

When will interference be constructive?

A

When waves are in phase and the same frequency

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47
Q

What must the path difference be for constructive interference?

A

nλ (where n is a whole number of wavelengths)

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48
Q

When will interference be destructive?

A

When waves are in antiphase and have the same frequency

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49
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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50
Q

What does it mean when waves are phase linked?

A

The waves have a constant phase difference

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51
Q

When are superposed waves easier to ‘see’?

A
  • the waves are of similar amplitude (↑ contrast between maxima and minima)
  • the waves have similar frequencies - otherwise the interference patterns create change so fast that they are difficult to detect
  • the waves have a constant phase difference i.e. they are phase linked
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52
Q

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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53
Q

What are coherent sources?

A

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

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54
Q

What are nodes?

A

On stationary waves, points that are always at equilibrium and 0 oscillation

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55
Q

What are antinodes?

A

On stationary waves, points of maximum oscillation

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56
Q

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

A

1/2 λ

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57
Q

How are stationary waves formed on a string?

A
  • vibrator moves up and down - sends travelling wave down cord
  • wave reflected at end, so 2 travelling waves overlap and interfere
  • has antinodes and nodes; distance between nodes = 1/2λ
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58
Q

What is the resonant frequency of a rubber band?

A

Where the band vibrates with large amplitude

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59
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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60
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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61
Q

When does resonance occur?

A

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

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62
Q

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

A
  • stationary - mπ (m = no. of nodes between the two particles)
  • travelling - 2πx/λ (x = distance apart)
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63
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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64
Q

What is a stationary wave?

A

Where energy is stored rather than transmitted - formed when 2 coherent waves travelling in opposite directions interfere to produce nodes and antinodes

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65
Q

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

A
  • ↑ tightness/tension
  • ↓ length of string
  • ↓ thickness of string
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66
Q

What does the 1st harmonic depend on?

A

1st harmonic frequency f depends on tension T in wire, its length l and its mass per unit length

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67
Q

What is the equation to calculate the frequency of the 1st harmonic?

A

f = 1/2l x √T/μ

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68
Q

What happens when the air at one end of the a 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

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69
Q

Where are the anti-nodes in an open pipe?

A

At both ends

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70
Q

Why are waves reflected at the ends of open pipes?

A

Air acts as a barrier outside

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71
Q

For the fundamental frequency in an open pipe, what is the pipe length?

A

λ/2

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72
Q

For an open pipe, which frequency is the first overtone?

A

2nd harmonic (2f₁)

73
Q

What is the frequency for the second harmonic in an open pipe?

A

2f₁

74
Q

For the second harmonic in an open pipe, what is the pipe length?

A

λ

75
Q

For an open pipe, which frequency is the second overtone?

A

3rd harmonic (3f₁)

76
Q

What is the frequency for the third harmonic in an open pipe?

A

3f₁

77
Q

For the third harmonic in an open pipe, what is the pipe length?

A

3λ/2

78
Q

Which harmonics can be obtained from an open pipe?

A

All of them

79
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

80
Q

Describe the amplitude of the particles in a closed pipe.

A

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

81
Q

For the fundamental frequency in a closed pipe, what is the pipe length?

A

λ/4

82
Q

For a closed pipe, which frequency is the first overtone?

A

3rd harmonic (3f₁)

83
Q

What is the frequency for the third harmonic in a closed pipe?

A

3f₁

84
Q

For the third harmonic in a closed pipe, what is the pipe length?

A

3λ/4

85
Q

For a closed pipe, which frequency is the second overtone?

A

5th harmonic (5f₁)

86
Q

What is the frequency for the fifth harmonic in a closed pipe?

A

5f₁

87
Q

For the fifth harmonic in a closed pipe, what is the pipe length?

A

5λ/4

88
Q

Which harmonics can be obtained in a closed pipe?

A

Odd harmonics

89
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

90
Q

What does the double slit interference pattern consist of?

A

Equidistant parallel fringes alternating between:

  • maxima (constructive interference)
  • minima (destructive interference)
91
Q

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

A

They interfere

92
Q

What does it mean if two sources are coherent?

A

They emit identical waves which start in phase

93
Q

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

A
  • use 2 coherent sources

* use single source with double slits

94
Q

What does it mean if two sources are coherent?

A

They have the same frequency, wavelength and synchronised phase changes

95
Q

What is the equation for the double slit interference pattern?

A

x = λD / a

96
Q

What does x stand for in x = λD / a?

A

Distance between adjacent fringes

97
Q

What does a stand for in x = λD / a?

A

Distance between slits (slit width)

98
Q

What does D stand for in x = λD / a?

A

Distance from slits to screen

99
Q

For small angles, what does sinθ equal?

A

tanθ

100
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

101
Q

How is light usually emitted?

A

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

102
Q

What is a monochromatic source?

A

A source of a single wavelength

103
Q

What is usually used as a monochromatic light source?

A

A sodium lamp

104
Q

Is there interference when two separate light sources are used?

A

Never

105
Q

What is fringe separation?

A

The distance between neighbouring bright fringes

106
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

107
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

108
Q

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

A

D↑ so distance between adjacent fringes increases

109
Q

What happens to the double slit interference pattern if the phase difference between 2 sources is changed to 180°?

A

The maxima will become minima and vice versa

110
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 ↑)

111
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

112
Q

How can you increase x in the young’s slits experiment?

A
  • ↑ D - measurement easier and more accurate but fringe intensity decreased
  • ↓ a - practical limit to this
  • ↑ wavelength
113
Q

Can mechanical waves interfere?

A

Yes

114
Q

What is diffraction?

A

When a wave passes through a gap and spreads out

115
Q

What happens to diffraction when the gap width ↓?

A

Diffraction ↑

116
Q

When is diffraction strongest?

A

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

117
Q

Why do waves passing through a single gap interfere?

A
  • only 1 slit but more than 1 wave
  • single slit can be thought of as large no. of sources next to each other
  • each ‘source’ produces coherent wave - overlap and interfere
118
Q

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

A
  • short λ (e.g. blue light) - narrow diffraction pattern

* long λ (e.g. red light) - broad diffraction pattern

119
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
120
Q

Difference between single and double slit pattern?

A
  • single slit - central max. fringe that is twice the width of the other fringes
  • double slit pattern has equally spaced fringes
121
Q

How many slits are on a diffraction grating?

A

1000s

122
Q

Which method produces a better diffraction pattern?

A

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

123
Q

What is a diffraction grating?

A

A set of slits for light waves to pass through

124
Q

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

A

m = 1/d

125
Q

Diffraction grating equation?

A

dsinθ = nλ

126
Q

What does d stand for in dsinθ = nλ?

A

Distance between slits

127
Q

What does θ stand for in dsinθ = nλ?

A

Angle to maxima

128
Q

What does n stand for in dsinθ = nλ?

A

Integer number to next bright fringe

129
Q

What does λ stand for in dsinθ = nλ?

A

Wavelength of light

130
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 be obtained

131
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

132
Q

What is the equation for the maximum number of orders?

A

n = d/λ

133
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

134
Q

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

A

n

135
Q

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

A

Diffraction grating

136
Q

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

A
  • double slits - fringes formed are slightly blurred → large errors
  • diffraction grating - images are clear and measurements accurate, also final result is an average of several calculations
137
Q

What can diffraction gratings be used for?

A

Analysis of spectra

138
Q

What is an optical fibre?

A

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

139
Q

What is refraction?

A

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

140
Q

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

A

It doesn’t refract

141
Q

What is a refractive index?

A

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

142
Q

What is the approximate refractive index of air?

A

1

143
Q

What is the equation for the refractive index of a material?

A

n = c/v

144
Q

What does each letter stand for in the equation n=c/v?

A

n = refractive index

c = velocity of light in vacuum

v = velocity of light in the medium

145
Q

What is the word 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

146
Q

What is Snell’s law?

A

n₁sinθ₁ = n₂sinθ₂

i.e. sin i / sin r = constant

147
Q

What are the two conditions for total internal reflection?

A
  • light passes from more to less dense medium

* angle of incidence > critical angle

148
Q

What is total internal reflection?

A

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

149
Q

What is the critical angle?

A

The limiting angle of incidence, as the angle of refraction cannot exceed 90°

150
Q

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

A

90°

151
Q

How is critical angle calculated in air?

A

sin c = 1 / n

152
Q

How does light travel along an optical fibre?

A

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

153
Q

What is an endoscope?

A

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

154
Q

What do endoscopes consist of?

A
  • a coherent bundle of fibres (lens system)

* an incoherent bundle of fibres (light delivery system)

155
Q

What happens if a fibre is bent too tightly?

A

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

156
Q

What can endoscopes be used to look at?

A

Digestive, respiratory and female reproductive systems

157
Q

What are the positives of endoscopes?

A

Can diagnose patients without an incision, often without anesthetic

158
Q

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

A

As long as θ is larger than the critical angle

i.e. sin θ > n of cladding ÷ n of core

159
Q

In medicine, what are the uses of optical fibres?

A
  • endoscopes

* lasers - burn tissue to heal wound

160
Q

What is a coherent bundle of fibres?

A

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

161
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 - ‘cross talk’
  • dispersion
162
Q

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

A

Using cladding

163
Q

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

A

Lower

164
Q

How is light sent down an optical fibre?

A

In ‘pulses’ or ‘bursts’

165
Q

How can a pulse be distorted in an optical fibre?

A
  • attenuation - some energy absorbed so pulse has smaller intensity
  • dispersion - causes pulse broadening
166
Q

What are the two types of dispersion?

A
  • material (chromatic) dispersion

* modal (multipath) dispersion

167
Q

How does multipath dispersion occur?

A

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

168
Q

How can multipath dispersion be decreased?

A
  • use monomode fibres with a core diameter of only a few wavelengths, so light travels via one path
  • cladding
169
Q

How can cladding help to reduce multipath dispersion?

A

Refractive index of cladding is only slightly lower than the refractive index of the core, so the critical angle is larger than is would be at a glass-air boundary → only small range of angles that can be transmitted

170
Q

What is the diameter of a typical mono-mode fibre?

A

10 micrometers (1-10 x 10⁻⁶ m)

171
Q

How can material dispersion be reduced?

A

Using monochromatic light (red will travel faster than blue)

172
Q

What colour of light should be used in an optical fibre?

A

Red - it travels faster

173
Q

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

A

Dispersion

174
Q

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

A

Blue

175
Q

Why is blue refracted more than red in a prism?

A

Blue light travels more slowly in glass than red light

176
Q

When is pulse distortion more of a problem?

A

When the pulses are very short and close together

177
Q

When are single fibres used?

A

In communications

178
Q

When are bundles of fibres used?

A

In endoscopes

AQA A Level Physics (10 decks)

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