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3.3 Waves Flashcards

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1
Q

What is a progressive wave?

A

A wave that transfers energy without transferring material and is made up of particles of a medium oscillating

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2
Q

What is amplitude?

A

A wave’s maximum displacement from its equilibrium position, in metres (m)

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3
Q

What is frequency?

A

The number of complete oscillations passing through a specific point per second, in Hertz (Hz)

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4
Q

What is wavelength?

A

The length of a complete oscillation (distance between successive peaks/troughs), in metres (m)

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5
Q

What is wave speed?

A

The distance travelled by a wave per unit time, in m/s

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6
Q

What is a phase?

A

The position of a certain point on a wave cycle, in radians, degrees or fractions of a cycle

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7
Q

What is phase difference?

A

How much a particle/wave lags behind another particle/wave, in radians, degrees or fractions of a cycle

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8
Q

What is a period?

A

The time taken for one full oscillation, in seconds (s)

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9
Q

What is a transverse wave?

A

A sinusoidal wave where the oscillation of particles/fields is perpendicular to the direction of energy transfer

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10
Q

What type of wave are EM waves?

A

Transverse

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11
Q

What speed do EM waves travel in a vacuum?

A

3x10^8 m/s

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12
Q

What are examples of demonstrating a transverse wave?

A

Shaking a slinky vertically or attaching string to a signal generator

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13
Q

What is a longitudinal wave?

A

A wave where the oscillation of particles is parallel to the direction of energy transfer

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14
Q

What do longitudinal waves consist of?

A

Compressions and rarefactions

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15
Q

Can longitudinal waves travel in a vacuum?

A

No

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16
Q

What type of wave is sound?

A

Longitudinal

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17
Q

How can you demonstrate a longitudinal wave?

A

Pushing a slinky horizontally

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18
Q

What is polarisation?

A

When the orientation of a transverse wave is specified so that waves of that orientation only are allowed though a medium

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19
Q

What type of waves can be polarised?

A

Transverse

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20
Q

What does polarising a wave do?

A

Make it so the wave can only oscillate in one plane

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21
Q

How is polarisation evidence for the nature of transverse waves?

A

It can only occur if a wave’s oscillations are perpendicular to its direction of energy transfer, as they are in transverses waves

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22
Q

What are examples of applications of polarisation?

A

Polaroid sunglasses and the alignment of aerials for transmission and reception

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23
Q

How do polaroid sunglasses work?

A

They only allow oscillations in the plane of the filter, making it easier to see by reducing glare by blocking partially polarised light reflected from other surfaces

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24
Q

How is polarisation used for transmitting and receiving TV and radio signals from aerials?

A

The signals are polarised by the orientation of the rods on the transmitting aerial, so the receiving aerial must be aligned in the same plane of polarisation to receive the signal at full strength

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25
Q

What is superposition?

A

Where the displacements of two waves are combined as they pass each other, with the resultant displacement being the sum of each wave’s displacement

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26
Q

What are the 2 types of interference that can occur during superposition?

A

Constructive interference and destructive interference

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27
Q

What is constructive interference?

A

Interference that occurs when 2 waves have displacement in the same direction

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28
Q

What is destructive interference?

A

Interference that occurs when one wave has positive displacement and the other has negative displacement

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29
Q

When does total destructive interference occur?

A

If two waves have equal but opposite displacements

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30
Q

How are stationary waves formed?

A

When two coherent waves of opposite directions with the same amplitude meet, they superpose, constructively interfering to form antinodes where the waves meet in phase, and destructively interfering to form nodes where the waves are completely of out phase

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31
Q

What are antinodes?

A

Regions of maximum amplitude

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32
Q

What are nodes?

A

Regions of no displacement

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33
Q

What is an example of a way stationary waves can be formed?

A

Using a string fixed at one end and fixed to a driving oscillator at the other end

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34
Q

How are stationary waves formed on a string which is fixed at one end and fixed to a driving oscillator at the other end?

A

A wave travelling down the string from the oscillator will be reflected at the fixed end and travel back along the string causing superposition, as the two waves are coherent and have the same amplitude, but opposite direction, making a stationary wave

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35
Q

What is the fundamental mode of vibration also known as?

A

The first harmonic

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36
Q

How is the first harmonic formed?

A

A stationary wave at the lowest possible frequency

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37
Q

What does the first harmonic consist of?

A

Two nodes and an anitnode

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38
Q

How is wavelength shown on a stationary wave?

A

The distance between adjacent nodes/antinodes is half a wavelength

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39
Q

What does μ​ represent in the first harmonic equation?

A

The mass per unit length

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40
Q

What is a harmonic?

A

The multiple of the fundamental modes of vibration

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41
Q

What are 2 examples of stationary waves?

A

Stationary microwaves - formed by reflecting a microwave beam at a metal plate
Stationary sound waves - formed by placing a speaker at one end of a closed glass tube, lay powder across bottom of tube, powder will be shaken at the antinodes and settle at nodes

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42
Q

How can nodes and antinodes be found in stationary microwaves?

A

Using a microwave probe

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43
Q

How can the speed of sound in air be found using stationary sound waves?

A

Place speaker at one end of closed glass tube
Lay powder across bottom of tube
Powder shaken at antinodes and settle at nodes
Wavelength found by doubling distance between 2 adjacent nodes/antinodes
Use wave speed formula

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44
Q

What is path difference?

A

The difference in distance travelled by two waves

45
Q

What is coherence?

A

The same frequency and wavelength with a fixed phase difference

46
Q

What is an example of a coherent and monochromatic light source?

A

Laser

47
Q

What does Young’s double slit experiment demonstrate?

A

Interference of light from two sources

48
Q

Describe Young’s double slit experiment.

A

Shine a coherent light source through 2 slits about the same size as the wavelength of the laser light so the light diffracts
Each slit acts as a coherent point source making an interference pattern of bright and dark fringes

49
Q

How are bright fringes formed?

A

Where the waves meet in phase and constructively interfere (path difference is whole number of wavelengths)

50
Q

How are dark fringes formed?

A

Where the waves meet completely out of phase and destructively interfere (path difference is a whole number and a half wavelengths)

51
Q

What is the formula for the phase difference for constructive interference?

A

(2n)π

52
Q

What is the formula for the phase difference for destructive interference?

A

(2n+1)π

53
Q

What is the formula for the path difference for constructive interference?

A

(n)λ

54
Q

What is the formula for the path difference for destructive interference?

A

(n+1/2) λ

55
Q

What does using a white light source with double slits show?

A

A wider maxima and a less intense diffraction pattern with a central white fringe, with alternating bright fringes which are spectra, with violet being closest to the central maximum and red furthest

56
Q

What are safety precautions that must be followed when using lasers?

A
  • Wear laser safety goggles
  • Don’t shine the laser at reflective surfaces
  • Display a warning sign
  • Never shine the laser at a person
57
Q

How can you demonstrate interference with sound waves?

A

Use two speakers connected to the same signal generator, and measure the intensity of the waves can be measured using a microphone to find the maxima (bright fringes) and minima (dark fringes)

58
Q

How did Young’s double slit experiment provide evidence for the wave nature of light (and disprove previous theories)?

A

Diffraction and interference are wave properties, so proved that EM radiation must act as a wave
There were theories that suggested light was formed of tiny particles
Knowledge and understanding of any scientific concept changes over time in accordance to the experimental evidence gathered​ by the scientific community

59
Q

What is diffraction?

A

The spreading out of waves when they pass through or around a gap

60
Q

At what size of the gap does the greatest diffraction occur?

A

The same size as the wavelength

61
Q

What does a gap smaller than the wavelength do to the diffraction?

A

Less noticeable diffraction as most waves are reflected

62
Q

What happens when a wave meets an obstacle?

A

There is diffraction around the edges, the wider the obstacle (compared to the wavelength) the less diffraction

63
Q

Describe the appearance of the diffraction pattern from a single slit using monochromatic light.

A

Interference pattern of bright and dark fringes, with a bright central fringe double the width of all other fringes, with alternating dark and bright fringes on either side

64
Q

Describe the appearance of the diffraction pattern from a single slit using white light.

A

A central white maximum with alternating bright fringes which are spectra, with violet being the closest to the central maximum and red furthest away

65
Q

Why do you get a spectrum of colour in the diffraction pattern of white light through a single slit?

A

The different wavelengths of light are all diffracted by different amounts

66
Q

What is the effect of increasing the slit width?

A

The amount of diffraction decreases, so the central maximum narrows and its intensity increases

67
Q

What is the effect of increasing the light wavelength through a single slit?

A

The amount of diffraction increases as the slit is closer in size to the light’s wavelength, so the central maximum widens and decreases in intensity

68
Q

What is a diffraction grating?

A

A slide containing many equally spaced slits very close together

69
Q

What is the difference between the interference pattern when using a diffraction grating instead of a double slit with monochromatic light and why?

A

Much sharper and brighter because there are many more rays of light reinforcing the pattern

70
Q

Why is it more accurate to measure slit widths using monochromatic light through a diffraction grating instead of a double slit?

A

There are many more rays of light reinforcing the pattern, making the fringes brighter and sharper, meaning measuring fringe widths is much more accurate as they are easier to take

71
Q

What is the zero order line?

A

The ray of light passing through the centre of a diffraction grating

72
Q

What are the lines either side the zero order line?

A

The first order lines (follows this pattern next to these)

73
Q

How do you derive the diffraction grating formula?

A
  1. Considering the ​first order maximum​, where the​ path difference between two adjacent rays of light is one wavelength, the angle between the normal to the grating and the ray of light ​θ
  2. A right angle triangle is formed, with side lengths d and λ, the upper angle in the triangle is θ
  3. By using trigonometry we can see that for the first maximum sin θ = λ/d, (as sin ​θ =Opp/Hyp​) which rearranges to dsin θ = λ, (for the first order)
  4. Replace λ with nλ to get: d sinθ = nλ
74
Q

What are examples of applications of diffraction gratings?

A

Line absorption spectra from splitting up light from stars - can be used to show elements present in star
X-ray crystallography - x-rays directed at a thin crystal sheet acting as a diffraction grating to form a diffraction pattern as the wavelength of the x-rays is similar in size to the gaps between the atoms - can be used to measure atomic spacing in certain materials

75
Q

What is the refractive index of a material?

A

The property which measures how much it slows down light passing through it

76
Q

What do ‘c’ and ‘cs’ represent in the refractive index equation?

A

c = speed of light in a vacuum = 3x10^8 m/s
cs = speed of light in the medium

77
Q

What is the refractive index of air (approximately)?

A

1

78
Q

What is an optically dense material?

A

A material with a high refractive index

79
Q

What is refraction?

A

When a wave enters a different medium it changes direction, either towards or away from the normal depending on the material’s refractive index

80
Q

What is Snell’s law of refraction for a boundary?

A

𝑛1sin𝜃1 = 𝑛2sin𝜃2

81
Q

In what direction does a light ray bend when travelling from a less optically dense material to a more optically dense material?

A

Bends towards the normal as it slows down

82
Q

In what direction does a light ray bend when travelling from a more optically dense material to a less optically dense material?

A

Bends away from the normal as it speeds up

83
Q

What is the equation for total internal reflection (critical angle)?

A

sin 𝜃𝑐 = 𝑛2/𝑛1

84
Q

What is the critical angle?

A

The angle of incidence when angle of refraction is 90 degrees

85
Q

How does total internal reflection occur?

A

When the angle of incidence is greater than the critical angle and the incident refractive index is greater than the refractive index of the material at the boundary

86
Q

What is an application of TIR?

A

Fibre optics

87
Q

What is the purpose of fibre optics?

A

Use total internal reflection to send high speed light signals over large distances

88
Q

What are fibre optics used in?

A

Communication of Internet transmission
Medical imaging (e.g. endoscopes)

89
Q

What are the three main components that make up fibre optics?

A
  1. An optically dense core (e.g. plastic/glass)
  2. A lower optical density cladding surrounding the core
  3. An outer sheath
90
Q

What is the function of the cladding in fibre optic cable?

A

Protects core from damage and prevents signal degradation by increasing the critical angle within the core fibre to prevent adjacent fibres from touching each other and light escaping
However, light with low angles of incidence can escape the fibre

91
Q

Why does the cladding refractive index need to be lower than the core’s refractive index in fibre optic cable?

A

So that total internal reflection can occur

92
Q

What is modal dispersion?

A

Waves enter an optical fibre at slightly different angles, meaning the distance each beam has to travel is slightly different, leading to the beams reaching the end at different times causing pulse broadening, meaning that signal is lost to the cladding from the core

93
Q

Why is signal lost to the cladding from the core in fibre optic cable?

A

The waves in an optical signal enter an optical fibre at slightly different angles, meaning the distance each beam has to travel is slightly different, leading to the beams reaching the end at different times causing pulse broadening

94
Q

What is the principle of pulse absorption?

A

The more transparent the material that makes up the core, the less light is absorbed, so the farther a pulse with a given energy can travel

95
Q

What is the consequence of pulse absorption?

A

As light is absorbed, less of the pulse remains, to a point where it decays completely or can’t be received

96
Q

How is pulse absorption minimised in fibre optic cable?

A

By the use of a very pure transparent material as the core

97
Q

What is the principle of material dispersion?

A

Waves of different wavelengths travel at slightly different speeds through an optical fibre so reach the end of the fibre at slightly different times, causing pulse broadening
The use of monochromatic light fixes this

98
Q

Why is monochromatic light used in fibre optic cable?

A

So all the pulse travels at the same speed, so there is no pulse broadening into a spectrum (like for white light)

99
Q

What are electromagnetic waves formed of?

A

Alternating sinusoidal magnetic and electric fields travelling in phase and perpendicularly

100
Q

What field of an EM wave can be polarised?

A

The electric field

101
Q

What was Maxwell’s theory of light?

A

Made up of two different perpendicularly oscillating transverse sinusoidal waves: a vertically oscillating electric field and a horizontally oscillating magnetic field

102
Q

What is Maxwell’s formula for the speed of electromagnetic waves in a vacuum?

A

𝑐 =1/(√𝜇0𝜀0)

𝜇0 - the permeability of free space, relates to the magnetic flux density due to a current-carrying wire in free space

𝜀0 - the permittivity of free space, relates to the electric field strength due to a charged object in free space

103
Q

How did Hertz discover radio waves?

A

He had a spark gap with conductors on either side in a circuit opposite a copper wire detector with the same spark gap and conductors
Electromagnetic waves travelled through the air to induce a p.d. in the copper wire detector causing a spark to jump across the detector gap

104
Q

How did Hertz’s experiment support that electromagnetic waves are transverse?

A

It was able to polarise the radio waves by rotating the detector so that the intensity of the sparks detected decreased to zero at 90 degrees, and increased to maximum rotating to 180 degrees

105
Q

How was Hertz able to determine the speed of radio waves from his experiment?

A

He created a stationary wave by placing a reflective surface behind the detector, creating nodes, which he could measure the distance between to find the wavelength

The frequency could be calculated from the known laws of electricity

106
Q

What was the setup of Fizeau’s experiment?

A

Light source reflecting from a partially reflecting mirror at 45 degrees towards a mirror with a rotating toothed wheel in between, then reflecting back through the partially reflecting mirror to an observer

107
Q

How did Fizeau determine the speed of light from his experiment?

A
  • When speed of rotation of the toothed wheel was low, light reflected back to observer, so light was fast enough to pass though gap in wheel and reflect back though same gap
  • After speed of rotation was slowly increased, light eventually couldn’t be seen by observer, so light wasn’t fast enough to pass back though gap after being reflected and is blocked by tooth next to gap
  • So by measuring the distance from the wheel and mirror, the frequency from the wheel’s rpm, and number of teeth in wheel he accurately determined the speed of light
108
Q

How do you increase the fringe spacing in a double slit experiment?

A

Decrease the separation of the double slits

109
Q

What are the affects of using light with a range of wavelengths in a single slit experiment?

A
  • Central maximum unchanged in width
  • Broader maxima/ range of angles for each maximum/order
  • Gradually getting broader/ more spread out for greater order maxima

Physics (16 decks)

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