Physics A-Level > Waves > Flashcards

Waves Flashcards

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1
Q

What is a progressive wave?

A
  • A wave that transfers energy from one point to another without transferring the medium itself
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2
Q

What are the properties of a progressive wave?

A
  • Displacement (x)
  • Amplitude (A)
  • Wavelength (λ)
  • Time period (T)
  • Frequency (f)
  • Speed (v)
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3
Q

What is amplitude of a wave?

A
  • The maximum displacement of a particle in the wave from its equilibrium position
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4
Q

What is wavelength of a wave?

A
  • The distance between points on successive oscillations of the wave that are in phase
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5
Q

What is time period of a wave?

A
  • The time taken for one complete oscillation or cycle of the wave
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6
Q

What is displacement of a wave?

A
  • The distance of a point on the wave from its equilibrium position
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7
Q

What is the frequency of a wave?

A
  • The number of complete oscillations per unit time
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8
Q

What is the speed of a wave?

A
  • The distance travelled by the wave per unit time
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9
Q

How do you calculate the speed of a wave?

A
  • v = fλ
  • For a wave at constant speed, as the wavelength increases, the frequency decreases
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10
Q

What is the phase difference?

A
  • A measure of how much a point or a wave is in front or behind another
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11
Q

How can the phase difference be found?

A
  • From the relative positive of the crests or troughs of two different waves of the same frequency
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12
Q

What does it mean if the waves are in phase and in antiphase?

A
  • When the crests or troughs are alligned, the waves are in phase
  • When the crest of one wave aligns with the trough of another, they are in antiphase
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13
Q

How is phase difference measured for in phase and in antiphase?

A
  • In phase is 360° or 2π
  • In antiphase is 180° or π
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14
Q

What is a mechanical wave and the different types?

A
  • In mechanical waves, particles oscillate about fixed points
  • The two types are transverse and longitudinal waves
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15
Q

How can you distinguish different types of waves?

A
  • Each type of wave can be distinguished by its direction of vibration relative to its direction of travel
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16
Q

What is a transverse wave?

A
  • A wave in which the particles oscillate perpendicular to the direction of the wave travel (and energy transfer)
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17
Q

What is a longitudinal wave?

A
  • A wave in which the particles oscillate parallel to the direction of the wave travel (and energy transfer)
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18
Q

What are the characteristics of transverse waves?

A
  • Transverse waves are shown by areas of crests (peaks) and troughs
  • All electromagnetic waves are transverse waves
  • Transverse waves can be polarised
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19
Q

What type of waves are electromagnetic waves?

A
  • All electromagnetic waves are transverse waves
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20
Q

What are the characteristics of longitudinal waves?

A
  • Longitudinal waves are shown by areas of compressions and rarefactions
  • Compressions are areas of increased pressure and rarefactions are areas of decreased pressure
  • Examples include sound waves and ultrasound waves
  • They cannot be polarised
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21
Q

How do longitudinal waves form?

A
  • Energy is transmitted through the wave
  • The particles in the medium vibrate as they are given energy
  • The compressions cause nearby particles to also vibrate with more energy
  • This produces a compression further along in the medium
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22
Q

How can waves be represented in real life?

A
  • Transverse waves can be represented using a rope, as the rope oscillates perpendicular to the direction of energy transfer
  • Longitudinal waves can be represented using a spring, as the spring oscillates parallel to the direction of energy transfer
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23
Q

What are some examples of transverse waves?

A
  • Electromagnetic waves (radio, visible light, UV)
  • Seismic (S) waves
  • Waves on a string
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24
Q

What are some examples of longitudinal waves?

A
  • Sound waves
  • Ultrasound waves
  • Seismic (P) waves
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25
What is polarisation?
- When oscillations occur in a single plane perpendicular to the direction of wave propagation
26
What type of waves can polarise occur in and why?
- Polarisation can only occur in transverse waves - This is because transverse waves oscillate in any plane perpendicular to the propagation direction - Longitudinal waves already oscillate in a single plane (parallel to wave propagation)
27
What happens when transverse waves are polarised?
- Oscillations are restricted to one direction - These vibrations are still perpendicular to the direction of propagation - A vertically polarised wave means only vertical oscillations can pass through
28
How can waves be polarised?
- Via a polariser or polarising filter - This only allows oscillations in a certain plane to be transmitted
29
How can light intensity be investigated for polarisation?
- If an unpolarised light source is placed in front of two identical polarising filters, A and B, with their transmission axes parallel, filter A will polarise the light in a certain axis and then all of the polarised light will pass through the filter B unaffected - In this case the maximum intensity of light is observed - As the polarising filter is rotated, the intensity of light will change depending on the angle B is rotated through - When A and B have their transmission axes perpendicular to eachother (rotated through 90° or 270°), filter A will polarise the light in a certain axis and then none of the polarised light will pass through filter B - In this case, the minimum intensity of light is transmitted
30
When is the maximum and minimum light intensity transmitted?
- Maximum: when both filter A and B have their transmission axes parallel - Minimum: when both filter A and B have their transmission axes perpendicular (rotated through 90° or 270°)
31
What happens to the intensity of light as it passes through a polarising filter in real life?
- The intensity of the unpolarised electromagnetic waves reduces after they pass through a polarising filter
32
What are some applications of polarisation?
- Polaroid sunglasses - Polaroid cameras
33
How is polarisation used in polaroid sunglasses and cameras?
- Glasses and cameras containing polarising filters have their transmission axes vertically oriented. This means the glasses do not allow any horizontally polarised light to pass through - When light is reflected from a reflective surface, it will undergo partial plane polarisation. This means that if the surface is horizontal, a proportion of the reflected light will oscillate more in the horizontal plane than in the vertical plane - Therefore, polaroid sunglasses and cameras are useful in reducing glare on the surface of the water - They also allow objects under the surface of the water to be viewed more clearly
34
What is a stationary wave?
- Stationary waves are produced by the superposition of two waves of the same frequency and amplitude travelling in opposite directions
35
What is the principle of superposition?
- If waves from two sources (or travelling by different routes from the same source) with the same frequency occupy the same region, then the total displacement at any one point is the vector sum of their individual displacements at that point
36
How are stationary waves produced?
- Two waves travelling in opposite directions along the same line with the same frequency superpose - Stationary waves are usually achieved by a travelling wave and its reflection
37
What are the differences between progressive and stationary waves?
- Stationary waves store energy while progressive waves transfer energy - Progressive waves, all points have the same amplitude, while in stationary waves, each point has a different amplitude - Stationary waves have nodes and antinodes, while progressive waves don't
38
How does the speed of the wave differ between progressive and stationary waves?
- The wave speed of a progressive wave is the speed at which the wave moves through a medium - In a stationary wave, each point on the wave oscillates at a different speed. The overall wave does not move
39
What is a stationary wave made up of?
- Nodes and antinodes
40
What are nodes and antinodes?
- A node is a region where there are no vibrations - An antinode is a region where the vibrations are at their maximum amplitude
41
What is the movement pattern of nodes and antinodes?
- Nodes and antinodes do not move along the string - Nodes are fixed and antinodes only move in the vertical direction
42
What is the phase difference of a stationary wave?
- The phase difference between two points on a stationary wave is either in phase or out of phase - Points between nodes are in phase with each other - Points that have an odd number of nodes between them are out of phase with each other - Points that have an even number of nodes between them are in phase with each other
43
What is constructive interference and how is it caused?
- When two waves superpose each other in phase - The peaks and troughs line up on both waves and the resultant wave has double the amplitude
44
What is destructive interference and how is it caused?
- When two waves superpose each other in anti-phase - The peaks on one wave line up with the troughs of the other - The resultant wave has no amplitude
45
What types of waves does superposition apply to?
- The principle of superposition applies to all types of wave
46
What is necessary for a stationary wave to form?
- Two waves must be travelling in opposite directions along the same line to superpose - They must have the same wavelength, frequency and amplitude
47
What happens when a stationary wave is formed?
- At the nodes: the waves are in anti-phase meaning destructive interference occurs. This causes the two waves to cancel each other out - At the antinodes: the waves are in phase meaning constructive interference occurs. This causes the waves to add together
48
What are some examples of stationary waves?
- Stretched strings - Microwaves - Sound waves
49
Explain the characteristics of stationary waves along a stretched string
- Vibrations caused by stationary waves on a stretched string produce sound - At specific frequencies, known as resonant frequencies, a whole number of half-wavelengths will fit on the string - As the resonant frequencies of the oscillator are achieved, stationary waves with different numbers of nodes and antinodes are formed
50
Explain the characteristics of stationary waves formed via microwaves
- A microwave source can be placed in line with a reflecting plate and a small detector between the two - The reflector can be moved back and forth to vary the stationary wave pattern formed
51
Explain the characteristics of stationary waves via sound waves
- Sound waves can be produced as a result of the formation of stationary waves inside an air column
52
What are harmonics?
- Harmonics are the different wave patterns that stationary waves have - These depend on the frequency of the vibration and the situation in which they are created
53
How can harmonics be observed?
- Harmonics can be observed on a string with two fixed ends - As the frequency is increased, more harmonics begin to appear
54
What is the first harmonic?
- When a stationary wave, such as a vibrating string, is fixed at both ends, the simplest wave pattern is a single loop made up of two nodes and an antinode (a node at both ends) - This is called the first harmonic, or the fundamental frequency
55
What is the fundamental frequency of a wave?
- The frequency at which the first harmonic is observed - This is when a stationary wave is produced with a single loop made up of two nodes and an antinode
56
What is the frequency of the first harmonic?
- f1 = V / λ - f1 = 1/2L x (sqrt( T / μ ))
57
What is the wavelength of the first harmonic?
- The total length of the wave between two fixed points is L - The wavelength is hence 2L (one single loop between both points)
58
What is the speed of a wave travelling along a string with two fixed ends?
- V = sqrt( T / μ )
59
How does the observed wave change as you increase harmonics?
- The nth harmonic has n antinodes and n+1 nodes
60
What do surface water waves show us?
- Clearly shows the effects of superposition - There are areas of zero displacement, where the water is flat - There are areas of increased displacement, where the water waves are higher
61
What is necessary for waves to undergo interference?
- The waves must be coherent - This means they must have the same frequency and a constant phase difference
62
What does coherence mean?
- Coherence occurs when waves have the same frequency and a constant phase difference
63
What are some examples of interference with coherent light sources?
- Monochromatic laser light - Sound waves from two nearby speakers emitting sound of the same frequency
64
What is path difference?
- The difference in distance travelled by two waves from their sources to the point where they meet
65
What is the difference between path difference and phase difference?
- Path difference compares the amount of progress made by waves along a path - Phase difference compares the distance between the phases (peaks and troughs) of coherent waves
66
What is the path difference between two waves used for?
- The path difference between two coherent waves determines whether there is constructive or destructive interference where they meet - Path difference is expressed in multiples of wavelength
67
How do we figure out if there is constructive or destructive interference?
- Constructive interference: path difference = nλ (whole number wavelength) - Destructive interference: path difference = (n = 1/2)λ (half wavelength)
68
What is a wavefront diagram?
- A diagram used to show wave behaviour - A curved line represents each wavefront (peak or trough) - This shows the interference between waves more clearly
69
What piece of equipment can we use to show wave patterns and why?
- Lasers are the ideal piece of equipment to analyse diffraction and intensity patterns because they form light that is: - Coherent (constant phase difference and frequency) - Monochromatic (have the same wavelength)
70
What does the diffraction pattern made up by a laser look like?
- Areas of constructive interference - the bright strips or fringes - Areas of destructive interference - the dark fringes
71
Why are lasers better than filament bulbs for showing wave patterns?
- Lasers produce coherent and monochromatic light - Filament bulbs produce non-coherent light
72
What precautions should be taken when working with lasers?
- Never look directly at a laser or its reflection - Don't shine the laser towards a person - Wear laser safety goggles - Stand behind the laser
73
How can we demonstrate two-source interference?
- Using two speakers emitting a coherent sound - Using microwaves or other electromagnetic waves
74
What do we see when we investigate two-source interference from sound waves?
- Constructive interference: occurs when the compressions and rarefactions from each wave line up and the sound appears louder - Destructive interference: occurs when a compression from one wave lines up with a rarefaction from the other and vice versa. The two waves cancel each other out, so no sound is heard
75
What do we see when we investigate two-source interference from microwaves?
- The detector picks up a maximum amplitude / intensity in regions of constructive interference - The detector picks up no zero amplitude (no signal) in areas of destructive interference
76
How is the intensity of a wave related to its amplitude?
- The intensity of a wave is proportional to the energy transferred by the wave - The energy transferred is proportional to the square of the amplitude - Hence, the intensity of a wave is proportional to the square of the amplitude - I ∝ A²
77
What are the two methods of carrying out Young's double slit experiment?
- The interference of two coherent wave sources - A single wave source passing through a double slit
78
What is the typical set up for Young's double slit experiment using a laser?
- The laser light source is placed behind a single sit - So the light is diffracted, producing two light sources at slits A and B - The light from the double slits is then diffracted, producing a diffraction pattern made up of bright and dark fringes
79
What do the fringes from Young's double slit experiment show us?
- A bright fringe (maxima) is where constructive interference has taken place. This has the maximum intensity and amplitude - A dark fringe (minima) is where destructive interference has taken place. This has a minimum or zero intensity and amplitude
80
How is path difference used in Young's double slit experiment?
- When the two waves from S1 and S2 (slit 1 and 2) interfere, the resultant wave depends on the path difference between the two waves from each slit - If the path difference is a whole integer value of the wavelength, there will be constructive interference - If the path difference is a half value of the wavelength, there will be destructive interference
81
How do you calculate the space between consecutive fringes?
- W = λD / s - W = fringe width (distance between successive bright fringes) - λ = wavelength of source - D = distance between double slit to the screen - s = distance between centres of the slit
82
What happens when a white light source is used in Young's double slit experiment?
- The central maximum is white because each wavelength interferes here constructively - There are two dark narrow destructive fringes on either side - Each maximum is of roughly equal length - All other maxima are composed of a spectrum - As you get further away from the central maxima, the wavelengths of blue observed decrease and the wavelengths of red observed increase
83
What colours are observed when a white light source is used in Young's double slit experiment?
- The shortest wavelength (violet / blue) appears nearest to the central maxima as it is diffracted the least - The longest wavelength (blue) appears further away from the central maxima - The colours look blurry and further away from the central maxima, the fringe spacing gets so small that the spectra eventually merge without any space between them
84
What order of scientists made discoveries regarding EM radiation?
- Isaac Newton (1672) - Christiaan Huygens (1678) - Thomas Young (1801) - James Clerk Maxwell (1862) - Albert Einstein (1905)
85
What did Isaac Newton propose about EM radiation?
- Newton proposed that visible light is a stream of microscopic particles called corpuscles - These corpuscles could not explain interference or diffraction effects, therefore, the view of light as a wave was adopted instead
86
What did Christiaan Huygens propose about EM radiation?
- Huygens came up with the original wave theory of light to explain the phenomena of diffraction and refraction - This theory describes light as a series of wavefronts on which every point is a source of waves that spread out and travel at the same speed as the source wave - These are known as Huygens wavelets
87
What did Thomas Young propose about EM radiation?
- Young devised the famous double-slit experiment - This provided experimental proof that light is a wave that can undergo constructive and destructive interference
88
What did James Clerk Maxwell propose about EM radiation?
- Maxwell showed that electric and magnetic fields obeyed the wave equation. This means that light was simply waves made up of electric and magnetic fields travelling perpendicular to one another - Later, Maxwell and Hertz discovered the full EM spectrum
89
What did Albert Einstein propose about EM radiation?
- Einstein discovered that light behaves as a particle (photoelectric effect) - He described light in terms of packets of energy called photons - The scientific community came to understand that light behaves as both a wave and a particle (wave-particle duality)
90
What is diffraction?
- Diffraction is the spreading out of waves after they pass through a narrow gap or around an obstruction
91
When are diffraction effects most noticeable?
- When the wavelength of the wave and the width of the gap it passes through are similar in size - For light, this effect becomes significant with very narrow slits - As the gap size increases, compared to the wavelength, the waves spread out less after they pass through the gap
92
What diffraction pattern is produced by light passing through a single slit?
- A series of light and dark fringes on a screen - The bright fringes are areas of maximum intensity, produced by the constructive interference of each part of the wavefront as it passes through the slit - The dark fringes are areas of zero or minimum intensity, produced by the destructive interference of each part of the wavefront as it passes through the slit
93
What do the fringes look like when blue light is passed through a single slit?
- The central maximum is much wider and brighter than the other bright fringes - On either side of the wide central maximum are much narrower and less bright maxima - Each maxima will be followed by a dark fringe of zero intensity - Moving away from the central maxima on each side, the intensity of each bright fringe decreases
94
What is the difference in the central maximum produced between blue light passed through a single slit and blue light passed through a double slit?
- The central maximum from a single-slit is much wider and brighter than that of a double-slit
95
What does the central maximum look like when white light is passed through a single-slit?
- The central maximum is bright white because constructive interference from all colours happens here - Much brighter and wider than other bright fringes - Much wider than that of the double-slit diffraction pattern - All the other maxima are composed of a spectrum
96
What do the fringes look like when white light is passed through a single-slit?
- Separate diffraction patterns can be observed for each wavelength of light - The shortest wavelength (blue) would appear nearest to the central maximum because it is diffracted the least - The longest wavelength (red) would appear furthest from the central maximum because it is diffracted more - The colours look blurry, and further away from the central maximum, the fringe spacing gets so small that the spectra eventually merge without any space between them
97
How do the wavelengths observed change as we move away from the central maximum when white light is passed through a single-slit?
- As the maxima move further away from the central maximum, the wavelengths of blue observed decrease and the wavelengths of red observed increase
98
What is the intensity pattern when white light is diffracted through a single-slit?
- The central maximum is equal in intensity to that of monochromatic light - The non-central maxima are wider and less intense - The fringe spacing between the maxima gets smaller - The amount of red wavelengths in the pattern increases with increasing maxima - The amount of blue wavelengths in the pattern decreases with increasing maxima
99
How does the Young's double-slit experiment relate to single-slit diffraction to form an intensity pattern?
- In Young's double-slit experiment, light passing through the two slits interferes, but each slit also diffracts light like a single slit - The double-slit interference pattern has equally spaced intensity peaks with maxima of equal intensity - The single-slit intensity pattern has a distinctive central maximum and subsequent maxima at lower intensity - Together, the combined intensity pattern has equally spaced bright fringes, but now within a single-slit envelope
100
What happens when you change the wavelength of light passing through a single-slit?
- When the wavelength of the light passing through the gap is increased, the wave diffracts more - This increases the angle of diffraction of the waves as they pass through the slit. Hence, the width of the bright maxima also increases - Red light (longest wavelength) will produce a diffraction pattern with wide fringes - Blue light (shortest wavelength) will produce a diffraction pattern with narrow fringes
101
What happens if you change the blue light laser to a red light when passing through a single-slit?
- There is much more diffraction as the waves pass through the slit - Hence, the fringes in the intensity pattern would be much wider
102
What happens if you change the slit width during single-slit diffraction?
- If the slit was made narrower, the angle of diffraction is greater so the waves spread out more beyond the slit - The intensity of the maxima would decrease. The width of the central maximum would increase and the spacing between fringes would be wider - The intensity pattern would be stretched horizontally
103
What is a diffraction grating?
- A piece of optical equipment that also creates a diffraction pattern when it diffracts: - Monochromatic light into bright and dark fringes - White light into its different wavelength components
104
What does a diffraction grating look like?
- A diffraction grating consists of a large number of very thin, equally spaced parallel slits carved into a glass plate
105
Why are diffraction gratings useful?
- Diffraction gratings are useful because they create a sharper pattern than a double-slit - This means their bright fringes are narrower and brighter while their dark regions are wider and darker
106
How do you calculate the location of maxima from a diffraction grating?
- d sinθ = n λ - d = distance between the slits of the grating (m) - θ = the angle of diffraction of the light of order n from the normal as it leaves the diffraction grating - n = the order of the maxima, the number of maxima away from the central
107
How is the size of a diffraction grating measured?
- The sizes are determined by the number of lines per millimetre or per metre - This is represented by the symbol N
108
What is the angular separation in a diffraction grating?
- The angle θ is taken from the centre (from the central maximum), meaning the higher order of n are at greater angles
109
What is the maximum angle of diffraction of a diffraction grating?
- The maximum angle of diffraction with which maxima can be seen is when the beam is at right angles to the diffraction grating - This means θ = 90, so sinθ = 1
110
What is the highest order of maxima visible from a diffraction grating?
- The highest order of maxima is n = d / λ - If the value obtained is a decimal, it must be rounded down
111
How does the path difference change as you go up maxima on a diffraction grating?
- When light passes through the slits of the diffraction grating, the path difference at the zeroth-order maximum - At the first order maxima, there is constructive interference, hence the path difference is λ - Therefore, at the nth order maxima, the path difference is equal to nλ
112
What is the derivation of the diffraction grate formula?
- Using trigonometry, the base side is λ (path difference between both waves) and the hypotenuse is the distance between slits of the grating (distance between source of both waves) - Hence sinθ = λ / d - For n=1: sinθ = λ / d - For n=2: sinθ = 2λ / d - For n: sinθ = nλ / d
113
What are diffraction gratings used for?
- Seperating light of different wavelengths with high resolution
114
What equipment are diffraction gratings used in?
- Spectrometers - X-ray crystalography - Monochromators - Optical fibre
115
How are diffraction gratings used in spectrometers?
- Used to analyse light from stars - Analyse the composition of a star - Measure red shift / rotation of stars
116
How are diffraction gratings used in X-ray crystallography?
- X-rays are directed at a thin crystal sheet which acts as a diffraction grating to form a diffraction pattern - This is because the wavelength of the X-rays is similar in size to the gaps between the atoms - The diffraction pattern can be used to measure the atomic spacing in certain materials
117
How are diffraction gratings used in monochrometers?
- Used to specifically analyse a wavelength emitted by molecules in diseased cells from a biopsy sample - Used to help excite molecules in a sample with a specific wavelength of light
118
How are diffraction gratings used in optical fibre?
- Diffraction gratings are used to select the optimum wavelength of light for use in optical fibre
119
What is the refractive index?
- The ratio of how fast light travels through a particular substance compared to the speed of light in a vacuum
120
What is refraction?
- The change in direction of a wave when it passes through a boundary between media of different densities
121
What is the cause of refraction?
- The change in direction is caused by a change in the speed of different parts of the wavefront as they hit the boundary
122
What is a medium?
- A transparent material
123
What are the conditions for different types of refraction?
- The more optically dense the material, the slower the waves travel and the smaller the angle of refraction. Hence, the light bends towards the normal - The less optically dense the material, the faster the waves travel and the larger the angle of refraction. Hence, the light bends away from the normal
124
What is the amount of refraction dependent on?
- The amount of refraction that takes place is determined by the difference between the angle of incidence and refraction of the waves at the boundary
125
What is the angle of incidence and refraction and how is it measured?
- They are measured from the normal line. The normal line is drawn at 90° to the boundary between the media - The angle of incidence is the angle between the initial incoming wave and the normal - The angle of refraction is the angle between the refracted wave and the normal
126
What happens if light passes from a less dense to a more dense medium?
- The light will travel at a slower speed - Hence, it will have a shorter wavelength - The light will bend towards the normal
127
What happens if light passes from a more dense to a less dense medium?
- The light will travel at a higher speed - Hence, it will have a longer wavelength - The light will bend away from the normal
128
What properties of the wave change as it refracts?
- When a wave refracts, its speed and wavelength change, but its frequency remains the same - This is noticeable by the fact that the colour of the wave does not change
129
What happens if the light ray is incident on the boundary at 90°?
- The wave is travelling along the normal - The wave passes straight through without changing direction - This is because the whole wavefront enters the boundary at the same time
130
How is the refractive index of a substance calculated?
- n = c / cs - n = refractive index - c = speed of light in a vacuum - cs = speed of light in the substance
131
What is always true for the refractive index?
- Since the speed of light in a substance will always be less than the speed of light in a vacuum, the value of n is always greater than 1
132
What is the refractive index in calculations?
- In calculations, the refractive index of air can be taken to be 1
133
What is Snell's law and what does it relate?
- Snell's law relates the angle of incidence to the angle of refraction at the boundary between two media - n1 sinθ1 = n2 sinθ2 - n1, n2 = refractive index of material 1 and 2 - θ1, θ2 = angle of incidence, angle of refraction
134
How does the critical angle relate to the refractive index?
- The larger the refractive index of a material, the smaller the critical angle - A larger n = a smaller θc
135
How is the critical angle reached in refraction?
- When light is shone at a boundary between a denser and a less dense material, different angles of incidence result in difference angles of refraction - As the angle of incidence is increased, the angle of refraction also increases - Until the angle of incidence reaches the critical angle
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What happens if the angle of incidence is equal to the critical angle?
- Angle of refraction = 90° - The refracted ray is refracted along the boundary between the two materials
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What happens if the angle of incidence is less than the critical angle?
- The ray is refracted and exits the material
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What happens if the angle of incidence is greater than the critical angle?
- The ray undergoes total internal reflection
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How do you find the critical angle of a material?
- The critical angle of material 1: - sin θc = n2 / n1 - This equation finds the critical angle of the material whos refractive index is the denominator
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When does total internal reflection occur?
- The angle of incidence within the denser material is greater than the critical angle - The incident refractive index, n, is greater than the refractive index of the material at the boundary (n1 > n2)
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What rule does total internal reflection follow?
- Angle of incidence = angle of reflection
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What rays are most likely to be internally reflected?
- Light rays inside a material with a higher refractive index are most likely to be totally internally reflected
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How do optical fibres work?
- Total internal reflection is used to reflect light along optical fibres - Light, that is normally monochromatic, refracts when it enters the optical fibre at one end - It undergoes repeated total internal reflection against the sides of the fibre until it reaches the other end - Where it is refracted back out
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What are optical fibres used for?
- Light signals travel long distances without losing information or speed - Used to transmit information and data
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What are some uses of optical fibres?
- Communication, such as telephone and internet transmission - Medical imaging, such as endoscopes
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What are the main components of optical fibres?
- An optically dense core tube, made of plastic or glass - A lower optically dense cladding surrounding the core - An outer sheath
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What is a step-index fibre and what happens at the boundary?
- A light ray is totally internally reflected down an optical fibre against the core-cladding boundary. Tota internal reflection only occurs when n(cladding) < n(core) - A step-index fibre is one where the refractive index of each component increases moving from the outside to the centre of the fibre (refractive index of core > refractive index of cladding)
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What is necessary in an optical fibre for total internal reflection to take place?
- The refractive index of the core must be greater than the refractive index of the cladding
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What is the role of the cladding in an optical fibre?
- Protect the thin core from damage and scratching - Prevent signal degradation through light escaping the core, which can cause information from the signal to be lost - It keeps the signals secure and maintains the original signal quality - It keeps the core separate from the other fibres, preventing information crossover
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What do material and modal dispersion cause?
- They both cause pulse broadening - This is where the pulses emerging from the fibre are longer than those entering it
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Why does material dispersion occur?
- When white light is used instead of monochromatic light inside an optical fibre, it is separated into all the colours on the spectrum - The white light is therefore dispersed, so the beam gets wider as it travels down the optical fibre
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How does violet refract to cause material dispersion within a white light spectrum?
- During material dispersion, white light is separated into all the colours on the spectrum - Each wavelength of light travels at the same speed in a vacuum, but at different speeds in a medium - Violet light has the shortest wavelength, so it travels the slowest in the fibre - This means its angle of incidence on the fibre boundary is smallest compared to the other colours - The angle of reflection is hence also smaller - This means it takes longer for the violet colour to travel down the fibre because it undergoes more reflections
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Why does modal dispersion occur?
- Occurs when the monochromatic light pulses in the optical fibre spread out - This is because each part of the wavefront has a different angle of incidence and consequently a different angle of reflection - So each part of the wavefront undergoes total internal reflection a different number of times - Hence, each part of the wavefront reaches the end of the fibre at a slightly different time
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When is modal dispersion most prominent and how is it reduced?
- Modal dispersion is most prominent when the core of the fibre is wider - This is because total internal reflection takes place more times - To prevent modal dispersion, the core needs to be very narrow
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What is absorption in optical fibres?
- The absorption of a signal in an optical fibre occurs when the fibre absorbs part of the signals energy - This reduces the amplitude of the signal, which can lead to a loss in the information transmitted
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What is pulse broadening in optical fibres?
- Pulse broadening is caused by modal and material dispersion - This can lead to the merging of pulses, which distorts information in the final pulse and decreases the amplitude of the signal
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How can you reduce absorption in optical fibres?
- Use an extremely transparent core - Use optical fibre repeaters so the pulse is regenerated before significant absorption has taken place
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How can you reduce pulse broadening in optical fibres?
- Use a core that is as narrow as possible to reduce possible differences in the path length of the signal - Use a monochromatic source so the speed of the pulse is constant - Use optical fibre repeaters so the pulse is regenerated before significant pulse broadening has taken place - Use a single-mode fibre, where only a single wavelength of light passes through the core, to reduce multipath modal dispersion
Physics A-Level
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