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A LEVEL - Physics > Section 3 - Waves > Flashcards

Section 3 - Waves Flashcards

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1
Q

What is a wave?

A

The oscillation of particles or fields.

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2
Q

What is a progressive wave?

A

A wave that carries energy from place to place without transferring any material.

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3
Q

What is a wave cycle?

A

One complete vibration of a wave.

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4
Q

What is the displacement of a wave and what is the unit?

A

How far a point on the wave has moved from its undisturbed position. Unit: metres

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5
Q

What is the amplitude of a wave and what is the unit?

A

The maximum magnitude of displacement. Unit: metres

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6
Q

What is the period of wave?

A

The time taken for a whole cycle (vibration) to pass a given point. Unit: seconds

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7
Q

What is the wavelength of a wave and what is the unit?

A

The length of one whole wave cycle, from crest to crest or trough to trough. Unit: metres

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8
Q

What is the frequency of a wave and what is the unit?

A

The number of cycles (vibrations) per second passing a given point. Unit: hertz

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9
Q

What is the phase of a wave?

A

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

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10
Q

What is the phase difference of a wave?

A

The amount one wave lags behind another.

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11
Q

What are the units for phase and phase difference?

A

Angles (degrees or radians) or as fractions of a cycle.

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12
Q

What are the symbols for displacement, amplitude, wavelength, period and frequency?

A
  • Displacement - x
  • Amplitude - A
  • Wavelength - Lambda
  • Period - T
  • Frequency - f
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13
Q

What is reflection?

A

When a wave is bounced back when it hits a boundary.

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14
Q

What is refraction?

A

When a wave changes direction as it enters a different medium.

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15
Q

What equation relates frequency and time period?

A

Frequency = 1 / Time period

f = 1 / T

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16
Q

What is the wave equation?

A

Wave speed = Frequency x Wavelength

c = f x lambda

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17
Q

What is c?

A

The speed of light in a vacuum - 3.0 x 10^8 m/s

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18
Q

What is the equation for wave speed?

A

Wave speed = Distance travelled / Time taken

c = d / t

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19
Q

What type of wave are EM waves?

A

Transverse

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20
Q

Give some examples of transverse waves.

A
  • EM Waves

* Water waves

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21
Q

What are the two types of graphs that can be drawn to show a transverse wave?

A

1) Displacement against distance along the path of a wave
2) Displacement against time for a POINT as the wave passes

(Note: 1 is just a standard graph of what a wave looks like. 2 is what happens to a specific point as a wave passes through it.)

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22
Q

Electromagnetic waves travel as vibrations through…

A

… magnetic and electric fields.

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23
Q

When looking at a graph representing a transverse wave, what must you look out for?

A

The label on the x axis. This may be distance or time, depending on what the graph is showing.

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24
Q

Describe the vibrations on a transverse wave.

A

At right angles to the direction of energy transfer.

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25
Q

Give some examples of a longitudinal wave.

A
  • Sound

* Pressure

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26
Q

What are the parts of a longitudinal wave?

A
  • Compressions

* Rarefactions

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27
Q

What are the anti-compressions in a longitudinal wave called?

A

Rarefactions

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28
Q

Do transverse and longitudinal waves require a medium?

A
  • Transverse - Usually no

* Longitudinal - Usually yes

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29
Q

How are longitudinal waves represented on a graph?

A
  • Displacement against time.

* This can it look like a transverse wave!

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30
Q

Describe the vibrations in a longitudinal wave.

A

Parallel to the direction of energy transfer.

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31
Q

What is a polarised wave?

A

A wave that only oscillates in one direction (e.g. only up and down).

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32
Q

Can transverse and longitudinal waves be polarised?

A
  • Transverse - Yes

* Longitudinal - No

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33
Q

Compare the vibrations in transverse and longitudinal waves.

A
  • Transverse - Perpendicular to the direction of energy transfer.
  • Longitudinal - Parallel to the direction of energy transfer.
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34
Q

What is polarisation?

A

Causing a transverse to only vibrate in one direction (e.g. up and down) usually by passing it through a polarisation filter.

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35
Q

What is some evidence for light being a transverse wave?

A

It can be polarised by reflection. A longitudinal wave could not do this, so light must be a transverse wave.

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36
Q

What is polarisation evidence for?

A

Which waves are transverse. For example, light can be polarised, so it must be transverse.

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37
Q

Describe how light was discovered to be a transverse wave.

A
  • In 1808, Malus discovered that light was polarised by reflection.
  • At the time, scientists thought light was a longitudinal wave, like sound.
  • In 1817, Young suggested that light must be a transverse wave, made of a vibrating electric and magnetic fields at right angles to the direction of energy transverse.
  • This explained why light could be polarised.
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38
Q

Why can light waves be polarised?

A

They are a mixture of different directions of vibration. This means that they can be polarised by allowing only some of these directions to pass through a filter.

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39
Q

What is a polarising filter?

A

A panel that polarised waves by only allowing a specific direction of vibration to pass through.

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40
Q

What happens when two polarising filters are arranged at right angles to each other?

A

No light will get through.

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41
Q

What happens in terms of polarisation when light is reflected off some surfaces?

A

It becomes partially polarised. This means some of it vibrates in the same direction.

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42
Q

How do polaroid sunglasses work?

A
  • Partially polarised light is reflected into a polarising filter at the correct angle.
  • This blocks out unwanted glare.
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43
Q

Give two examples of when wave polarisation is used.

A
  • Polaroid sunglasses

* TV and radio signals

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44
Q

How do TV and radio signals make use of wave polarisation?

A
  • Broadcasting aerial has rods, which emit polarised waves
  • TV aerials on homes have horizontal rods
  • These rods must be lined up in order to get maximum signal strength
  • The same thing happens with radio aerials
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45
Q

What is superposition?

A

When two or more waves pass through each other and their displacements combine.

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46
Q

What does the principle of superposition state?

A

When two or more waves cross, the resultant displacement equals the vector sum of the individual displacements.

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47
Q

Graphically, how do you superimpose waves?

A

Add the individual displacements at each point along the x-axis and then plot these.

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48
Q

What happens when a crest meets a crest (or a trough meets a trough) and what is this called?

A
  • Constructive interference

* The amplitude of the wave is increased (i.e. the crest or trough gets bigger).

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49
Q

What happens when a crest meets a trough of the same size and what is this called?

A
  • Destructive interference

* The displacements cancel themselves out.

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50
Q

What does it mean when two points on a wave are “in phase”?

A

They are both at the same point in the wave cycle. They are likely to be 360, 720, etc. out of phase.

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51
Q

What quantities are the same about points on a wave which are in phase?

A
  • Same velocity

* Same displacement

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52
Q

How many degrees is one complete wave cycle said to be?

A

360*

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53
Q

How many radians is one complete wave cycle?

A

2π radians

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54
Q

How many degrees is a radian?

A

180/π

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55
Q

What is the SI unit for angle?

A

Radian -> 1 radian is equal to 180/π.

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56
Q

How do you convert from degrees to radians?

A

Multiply by π/180.

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57
Q

How do you convert from radians to degrees?

A

Multiply by 180/π.

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58
Q

What is half a wavelength in degrees and radians?

A
  • 180*

* π radians.

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59
Q

What is 1/4 of a wavelength in degrees and radians?

A
  • 90*

* 1/2 π radians

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60
Q

What is 3/4 of a wavelength in degrees and radians?

A
  • 270*

* 3/2 π radians

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61
Q

What is a whole wavelength in degrees and radians?

A
  • 360*

* 2π radians

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62
Q

What is the phase difference of a vibrating particle?

A

The fraction of a cycle it has completed since the start of a cycle.

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63
Q

What a the phase difference between two particles?

A

Thee fraction of a cycle between the vibrations of the particles, measured in either degrees or radians.

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64
Q

What is the unit for phase difference?

A

Degrees or radians.

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65
Q

Waves with a phase difference of 0* or a multiple of 360* are said to be…

A

… in phase.

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66
Q

Waves with a phase difference of an odd number multiple of 180* are said to be…

A

… exactly out of phase.

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67
Q

When are two sources said to be coherent?

A

When they have the same:
• Wavelength
• Frequency
And have a fixed phase difference between them.

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68
Q

When are interference patterns most clear?

A

When the two sources are coherent (have the same wavelength and frequency and have a fixed phase difference between them).

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69
Q

What is path difference and when is it relevant?

A
  • How much further a wave has travelled compared to another
  • This is used when looking at the type of interference between two waves that will occur at a certain point (see diagram pg 27 of revision guide).
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70
Q

Assuming that two sources are coherent and in phase, at what path difference will constructive interference occur?

A

At a whole number of wavelengths.

Path difference = nλ

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71
Q

Assuming that two sources are coherent and in phase, at what path difference will destructive interference occur?

A

At a whole number of wavelengths and a half.

Path difference = nλ + 0.5λ

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72
Q

What is a stationary wave?

A

The superposition of two progressive waves with the same frequency (wavelength) moving in opposite directions.

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73
Q

Do stationary waves transmit energy?

A

No

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74
Q

Describe how the wave on a fixed piece of string (so it reflects at the end) changes with frequency.

A
  • At most frequencies, the pattern on the string is a jumble
  • If the vibration generator produces an exact number of waves in the time it takes a wave to get to the end and back, the original and reflected waves reinforce each other. This produces a stationary wave.
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75
Q

Describe how stationary waves in a string can be demonstrated.

A
  • Vibration generator is attached to a piece of string at one end, while the string is fixed at the other end.
  • The frequency of the generator is varied until a resonant frequency is found.
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76
Q

What is a node on a stationary wave?

A

Where the amplitude of the vibration is zero.

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77
Q

What is an antinode on a stationary wave?

A

Where the maximum amplitude of the wave is.

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78
Q

What are the sections of stationary wave on a string called?

A

Oscillating loops

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79
Q

What can be said about the number of wavelengths on a piece of string at a resonant frequency?

A

An exact number of half wavelengths fit onto the string.

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80
Q

What is it called when one, two and three loops of stationary wave are found on a string?

A

1 Loop = First harmonic
2 Loops = Second harmonic
3 Loops = Third harmonic

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81
Q

What is the first harmonic?

A
  • When the stationary wave is vibrating at the lowest possible resonant frequency.
  • One loop is on the string, with a node at each end.
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82
Q

At the first harmonic, what is the length of the section of string?

A

1/2 a wavelength of the wave

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83
Q

At the second harmonic, what is the length of the section of string?

A

1 wavelength

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84
Q

At the third harmonic, what is the length of the section of string?

A

1.5 wavelengths

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85
Q

Remember to revise harmonic diagrams.

A

Pg 28 of revision guide.

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86
Q

Name some ways in which stationary waves can be demonstrated.

A
  • Microwaves reflected off a metal wave + Probe
  • Powder in a tube of air
  • Vibration generator connected to a fixed string
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87
Q

Describe how microwaves can be used to demonstrate stationary waves.

A
  • Microwave transmitter is pointed at a metal plate, which reflects microwaves.
  • A probe connected to a loudspeaker or meter is moved between the transmitter and the plate to try and find nodes and antinodes.
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88
Q

Describe how sound can be used to demonstrate stationary waves.

A
  • Glass tube with a speaker at the end is set up
  • Lycopodium powder is laid along the bottom of the tube
  • The powder is shaken away from the antinodes and left undisturbed at the nodes
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89
Q

Compare the frequency of the first, second and third harmonic.

A
  • First = f
  • Second = 2f
  • Third = 3f
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90
Q

Which equation can be used to find the frequency of the nth harmonic on a piece of string?

A

f = c / λ

Where:
f = Harmonic frequency
c = Speed of wave on string
λ = The wavelength of the wave given in terms of the length of the string (e.g. first harmonic: λ = 2L)

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91
Q

In terms of wave speed and sting length, at what frequency is the first harmonic achieved?

A

f = c / 2L

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92
Q

In terms of wave speed and sting length, at what frequency is the second harmonic achieved?

A

f = c / L

93
Q

In terms of wave speed and sting length, at what frequency is the third harmonic achieved?

A

f = 3c / 2L

i.e. f = c / (2/3 L)

94
Q

What is the equation for phase difference in radians?

A

Phase difference (radians) = 2πd / λ

Where d = the distance apart of the particles in wavelengths (e.g. d might equal 1/4 λ if there is a quarter of a cycle difference)

95
Q

Describe an experiment used to find the resonant frequencies of a string.

A

1) Measure the mass and length of the string using a balance and ruler. Work out the mass per unit length (μ = M/L) in kg/m.
2) Set up the equipment as shown on pg 29 of the revision guide. This involves connecting a vibration generator (connected to a signal generator) to a piece of string attached to a pulley and some masses. Clamp the entire setup to the bench.
3) Measure the length (l) of the string between the vibration generator and the pulley. Work out the tension using (T = mg) where m is the mass of the masses on the end of the string.
4) Turn on the signal generator and adjust the frequency until the first harmonic is found.

96
Q

What are the first, second, third, etc harmonics known as collectively?

A

The resonant frequencies.

97
Q

Which factors during the stationary wave experiment may affect the resonant frequencies?

A
  • Length of the vibrating string
  • Tension in the string
  • Type of string (different μ)
98
Q

In the stationary wave experiment, what do the letters μ, Μ, L, T, m and g represent?

A
  • μ = Mass per unit length of string
  • Μ = Mass of the string
  • L = Length of vibrating string
  • T = Tension in the string
  • m = Mass of the masses in the end of the string
  • g = Gravitational field strength
99
Q

What is the unit for tension?

A

Newtons (N)

100
Q

Remember to revise the stationary waves experiment.

A

Pg 29 of revision guide.

101
Q

How can the length of the vibrating string in the stationary waves experiment be varied?

A
  • Keep the type of string and tension the same

* Move the vibration transducer towards or away from the pulley

102
Q

How can the tension in the string in the stationary waves experiment be varied?

A
  • Keep the string type and length the same

* Add or remove masses to vary tension

103
Q

How can the string type in the stationary waves experiment be varied?

A
  • Keep the vibrating string length and tension the same

* Use different string samples to vary μ

104
Q

How does string length affect the resonant frequency in the stationary wave experiment?

A
  • The longer the string, the lower the resonant frequency.

* Because the half wavelength at the resonant frequency is longer.

105
Q

How does the type of string affect the the resonant frequency in the stationary wave experiment?

A
  • The heavier (greater μ) the string, the lower the resonant frequency.
  • Because waves travel more slowly down the string. A lower wave speed, c, makes a lower frequency, f.
106
Q

How does tension affect the the resonant frequency in the stationary wave experiment?

A
  • The higher the tension, the higher the resonant frequency.

* Because waves travel more quickly on a taut string. A higher wave speed, c, makes a higher frequency.

107
Q

In the stationary wave experiment, what equation is used to give the FIRST harmonic frequency?

A

f = (1 / 2l) x root(T / μ)

Where:
l = String length (m)
T = Tension in string
μ = Mass per unit length of string (kg/m)

See page 29 of revision guide.

108
Q

Remember to revise the equation for the first harmonic frequency in the stationary wave experiment.

A

Pg 29 of revision guide

109
Q

What is diffraction?

A

The spreading out of waves when passing through a gap (or going around an object).

110
Q

What determines the amount of diffraction observed?

A

The wavelength of the wave compared to the size of the gap.

111
Q

When is diffraction most noticeable?

A

When the gap is the same size as the wavelength.

112
Q

How does a narrower gap affect diffraction?

A

It is increased.

113
Q

How does a smaller wavelength affect diffraction?

A

It is decreased.

114
Q

What happens in terms of diffraction when the gap is a lot bigger than the wavelength?

A

Diffraction is unnoticeable.

115
Q

What happens in terms of diffraction when the gap is a lot smaller than the wavelength?

A

The waves are mostly just reflected back.

116
Q

Describe what happens to the amount of diffraction with a set wavelength as the gap size changes from very large to very small.

A
  • Gap is a lot bigger than the wavelength -> Diffraction is unnoticeable
  • Gap is several wavelengths wide -> Noticeable diffraction
  • Gap size = Wavelength -> Maximum diffraction
  • Gap is smaller than the wavelength -> Waves are mostly just reflected
117
Q

When both are not in direct line of sight, why can sound be heard around a doorway, but light cannot be seen.

A

The doorway is a gap of a similar size to the wavelength of sound, so it diffracts to the listener. However, the gap is much larger than the wavelength of light, so the diffraction is not noticeable.

118
Q

What is produced when monochromatic light is shone through a single slit?

A

• A diffraction pattern.
This consists of:
• A bright central maximum
• Alternating dark and bright fringes on either side (of decreasing intensity)

119
Q

In order to observe a single-slit diffraction pattern, what light source must be used? Give an example.

A
  • Coherent and monochromatic
  • Similar wavelength to the slit width
  • E.g. A laser
120
Q

What causes the single-slit diffraction pattern?

A

The dark and bright fringes are due to destructive and constructive interference.

121
Q

What is monochromatic light?

A

Light all of the same wavelength (and so of the same colour).

122
Q

Describe and explain the diffraction pattern observed with white light passing through a single slit.

A
  • A single white slit is in the centre, with each bright fringe being a spectrum on either side (instead of clear bands)
  • This is because each wavelength of light is diffracted by a different amount
123
Q

In a single-slit white light diffraction pattern, what is the order of colours in each spectrum band and why?

A
  • Blue is on the inner side, while red is on the outer side

* This is because red light has a longer wavelength, so it diffracts more

124
Q

Remember to revise single-slit diffraction patterns.

A

See pg 30 of revision guide.

125
Q

What happens to each fringe in a single-slit diffraction pattern as you move from the central maximum?

A

The fringes become less bright.

126
Q

What is intensity of light?

A

The power per unit area.

127
Q

For monochromatic light, what does an increase in intensity mean?

A

There are more photons hitting the area per second (since each photon has the same energy).

128
Q

In a single-slit diffraction pattern, how does the width of the central maximum compare to the outer fringes?

A
  • It is twice as wide

* The outer fringes are all of the same width

129
Q

Name two monochromatic light sources.

A
  • Laser

* Vapour lamps and discharge tubes

130
Q

What are the conditions for coherent light sources?

A

They must emit light of:
• The same frequency
• A constant phase difference

131
Q

Do two light sources have to be in phase to be coherent?

A

No, as long as they have a constant phase difference.

132
Q

In single-slit diffraction, what can be said about the number of photons hitting the central maximum compared to the other bright fringes?

A

There are more photons hitting it per second.

133
Q

What is the single-slit equation?

A

W = 2Dλ/a

Where:
• W - Width of the central maximum
• D - Distance between the slit and screen
• λ - Wavelength
• a - Slit width
(All units in m)
134
Q

Which of the wave equations is not given on the equation sheet?

A

Single-slit interference

W = 2Dλ/a

135
Q

In single-slit diffraction, what is the effect of increasing slit width on the intensity and width of the central maximum and why?

A
  • Width is decreased
  • Intensity is increased
  • Due to less diffraction
136
Q

In single-slit diffraction, what is the effect of increasing wavelength on the intensity and width of the central maximum and why?

A
  • Width is increased
  • Intensity is decreased
  • Due to more diffraction
137
Q

What is needed to demonstrate two-source interference?

A

Two coherent sources.

138
Q

How can you ensure that two sources are coherent when demonstrating two-source interference with water or sound?

A

Connect both dippers/loudspeakers to the same vibrator/oscillator.

139
Q

What is another name for the experiment to demonstrate two source interference?

A

Young’s double-slit experiment

140
Q

In Young’s double slit experiment, what do the slits act as?

A

Two coherent sources of light

141
Q

Can a white light source be a coherent source?

A

No, due to the various frequencies of light in it.

142
Q

Remember to revise the set up for Young’s double slit experiment.

A

Pg 32 of revision guide.

143
Q

What is produced when monochromatic light is shone through double slits?

A

• A fringe pattern.
This consists of:
• Alternating bright and dark fringes of equal width and decreasing brightness

144
Q

What causes the double slit interference pattern?

A
  • Constructive interference causes the bright fringes -> Where the phase difference is a multiple of the wavelength
  • Destructive interference causes the dark fringes -> Where the phase difference is a multiple of the wavelength plus half a wavelength
145
Q

Why can the bands in the diffraction pattern only be called “fringes” in the double slit patter, not the single slit pattern?

A

Because they are all of the same width, unlike in the single slit pattern.

146
Q

What can be said about the phase difference at a bright fringe in the double slit interference pattern?

A

P.d. = nλ

Where n is an integer.

147
Q

What can be said about the phase difference at a dark fringe in the double slit interference pattern?

A

P.d. = nλ + 0.5λ

Where n is an integer.

148
Q

What is fringe separation?

A

The distance from the centre of a bright fringe to the centre of the next one.

149
Q

What is the danger of a powerful laser?

A

If you looked at the beam directly, your eye would focus it onto your retina, which would be permanently damaged.

150
Q

What are some safety precautions that must be taken when working with a laser?

A

1) Never shine the laser towards a person.
2) Wear laser safety goggles.
3) Avoid shining the beam at a reflective surface.
4) Have a warning sign on display.
5) Turn the laser off when it’s not needed.

151
Q

How can Young’s double slit experiment be adapted for microwaves?

A
  • Replace the laser and slits with 2 microwave transmitter cones attached to the same signal generator
  • Replace the screen with a receiver probe
  • Move the probe along where the screen was and you’ll get an alternating pattern of strong and weak signals
152
Q

What is the equation for Young’s double-slit experiment?

A

w = λD/s

Where:
w = Fringe spacing
λ = Wavelength
D = Distance from slits to screen
s = Slit separation
153
Q

In Young’s double slit experiment, what is the easiest way to get an accurate reading for ‘w’?

A

Measure several fringes and divide by the number of fringe widths between them.

154
Q

In Young’s double slit experiment, what must you be careful of when measuring several fringes?

A
  • When dividing to find ‘w’, remember to divide by the number of fringe WIDTHS between them, not the number of fringes.
  • e.g. 10 bright lines only have 9 fringe widths between them.
155
Q

Compare single and double slit diffraction patterns in terms of fringe widths and intensities.

A

Single slit:
• Widest central maximum + equal outer fringes
• Brightest central maximum + decreasing intensity of outer fringes
Double slit:
• All fringes of equal width
• Decreasing intensity of outer fringes

156
Q

In a double slit interference pattern, why does the intensity of the fringes decrease as you get further away from the central maximum?

A

Because it’s multiplied by the single slit diffraction pattern for either of the slits separately.

157
Q

Compare the double slit interference pattern for red and blue light.

A

The blue light creates a smaller fringe separation. This makes the pattern appear more compact.

158
Q

Describe and explain what is observed with double slit interference of WHITE light.

A
  • White central fringe - Every colour contributes at the centre
  • Inner fringes tinged with blue on the inside and red on the outer side - Red fringes are more spaced out than blue fringes.
  • After a few fringes, no clear fringe pattern - The different colour’s fringe patterns have all blended
159
Q

What was the importance of Young’s double slit experiment?

A
  • It was evidence for light interference and diffraction.
  • This was important in the debate between Newton’s particle (corpuscle) theory of light and Huygen’s wave theory of light.
  • It supported Huygen’s theory (even though the debate was raging again 100 years later).
160
Q

Explain what happens when Young’s double slit experiment is repeated with more slits.

A
  • The same shaped pattern is observed, except the bright bands are brighter and the dark bands are darker
  • This gives a sharper pattern
161
Q

What makes the pattern so sharp when monochromatic light is passed through a diffraction grating?

A

There are many beams reinforcing the pattern.

162
Q

What is the advantage of observing sharper lines in interference patterns?

A

It allows for more accurate measurements.

163
Q

In double slit interference, what conditions must be met in order for a pattern to be seen?

A
  • Each slit must be sufficiently narrow to diffract the light enough
  • The two slits must be close enough for the diffracted waves to overlap
164
Q

Explain simply why single-slit diffraction patterns are observed.

A

The waves from different points across the slit interfere to reinforce or cancel each other.
(See pg 84 of textbook for a very good explanation!)

165
Q

Explain why a diffraction grating produces several sharp lines.

A
  • Diffracted light waves from adjacent slits reinforce each other in certain directions only and cancel out in all other directions.
  • It works just like with double slit interference, except with many more slits.
  • More slits result in more sharp lines, so the are several distinct, sharp lines produced by a diffraction grating.
166
Q

With a diffraction grating, what is the central maximum line called?

A

The 0 order line

167
Q

Describe what the sharp lines produced by a diffraction grating are called.

A

• The central beam is called the zero order.
• The lines just either side are the 1st order (n=1).
• The next lines are the 2nd order, etc.
(See diagram pg 34 of the revision guide)

168
Q

What is a diffraction grating?

A

A plate with many closely spaced parallel slits ruled on it.

169
Q

What is the diffraction grating equation?

A

dsinθ = nλ

Where:
• d - Slit spacing (m)
• θ - Angle from normal
• n - Order
• λ - Wavelength (m)
170
Q

What must you be careful of when putting ‘d’ into the diffraction grating equation?

A

d is the slit spacing, not the number of slits per metre, which is how the data may be given.

171
Q

When given a grating with 300 slits per mm, what value of d is used?

A
  • 300 slits/mm = 300,000 slits/m

* Therefore, d = 1/300,000

172
Q

Remember to practise deriving the diffraction grating equation.

A

See pg 34 of revision guide + have a go.

173
Q

What effect does increasing wavelength have on the diffraction grating pattern?

A

It is more spread out.

174
Q

What effect does increasing slit separation have on the diffraction grating pattern?

A

It is more compact.

175
Q

How can you calculate the maximum order for a given diffraction grating and wavelength? Explain why this works.

A
  • θ can never be greater than 90. Therefore, the greatest value that sinθ can have is sin(90), which is 1.
  • So, replace sinθ with 1 in the equation.
  • This leaves d = nλ. Solve for n.
  • Round n DOWN to the nearest integer.
176
Q

When calculating the maximum order for a certain diffraction grating and a certain wavelength, what must you do to the value of n obtained?

A

Round it down to the next integer.

177
Q

What happens when white light is passed through a diffraction grating and why?

A

• Each order becomes a spectrum because the different colours of light are diffracted by different amounts.
• Blue on the inside, red on the outside -> Blue has a shorter wavelength, so it is diffracted less.
(See diagram pg 35 of revision guide)

178
Q

Give some uses of diffraction gratings.

A
  • Producing spectra to identify elements

* X-ray crystallography

179
Q

What is X-ray crystallography?

A
  • The wavelength of x-rays is similar to the spacing between atoms in crystalline solids.
  • So x-rays directed at a thin crystal form a diffraction pattern -> The crystal acts like a diffraction grating.
  • Looking at the diffraction pattern, the spacing of the atoms can be calculated.
180
Q

What was x-ray crystallography used for?

A

To discover the structure of DNA.

181
Q

What is the absolute refractive index of a material?

A
  • A measure of optical density

* A ratio of the speed of light in a vacuum compared to the speed of light in the material.

182
Q

When does light travel the fastest?

A

In a vacuum.

183
Q

Why does light slow down in materials?

A

It interacts with the particles in the material.

184
Q

What is an optically dense material?

A

One that slows light down a lot.

185
Q

What symbol is used for the speed of light in a vacuum?

A

c

186
Q

What symbol is used for the speed of light in a material?

A

cs (subscript s)

187
Q

What is relative refractive index?

A

The ratio of the speed of light in material 1 to the speed of light in material 2.

188
Q

What is the symbol for absolute refractive index?

A

n

189
Q

What is the symbol for relative refractive index?

A

1n2 (1 and 2 in subscript)

190
Q

What is the speed of light in a vacuum?

A

3.00 x 10^8 m/s

191
Q

What is the difference between absolute and relative refractive index?

A

• Absolute refractive index is the ratio of the speed of light in a vacuum compared to the speed of light in the material.
• Relative refractive index is the ratio of the speed of light in material 1 to the speed of light in material 2.
(NOTE: Absolute refractive index is just a case of the relative refractive index)

192
Q

What is the equation for absolute refractive index?

A

n = c/cs

NOTE: This is just a specific case of the equation for relative refractive index

193
Q

What are the equations for relative refractive index?

A

1n2 = c1/c2
or
1n2 = n2/n1

194
Q

In the exam, you are given n = c/cs. Practise deriving the two equations for relative refractive index from this.

A
  • n = c/cs
  • 1n2 = c1/c2 (It’s logical from the definition!)
  • 1n2 = n2/n1
195
Q

How many materials do absolute refractive index and relative refractive index refer to?

A
  • Absolute - Property of one material only.

* Relative - Property of the interface between two materials. It is different for every possible pair.

196
Q

What is the refractive index of air?

A

1

197
Q

In refractive calculations, how are air and vacuum perceived?

A

They are essentially the same since they both have a refractive index of about 1.

198
Q

What is the angle of incidence and what is the symbol?

A

The angle that incoming light makes to the normal.

Symbol: θ1

199
Q

What is the angle of refraction?

A

The angle that the refracted ray makes to the normal.

Symbol: θ2

200
Q

What must you be careful of when dealing with the angle of incidence and the angle of refraction?

A

They are measured from the NORMAL, not the boundary.

201
Q

Which was does light bend when it enters a more optically dense material?

A

Towards the normal.

202
Q

Which was does light bend when it enters a less optically dense material?

A

Away from the normal.

203
Q

What is Snell’s law?

A

n1 x sinθ1 = n2 x sinθ2

204
Q

What is the critical angle?

A

The angle of incidence at which the angle of refraction is 90* so that the light is refracted along the boundary.

205
Q

What is the equation for the critical angle?

A

sinθc = n2/n1 = 1n2

206
Q

Derive the equation for the critical angle.

A
  • At the critical angle, the angle of refraction is 90*
  • So sinθ2 = sin90 = 1
  • n1 x sinθc = n2 x 1
  • sinθc = n2/n1
207
Q

When drawing diagrams in semi-circle blocks, what is it important to remember?

A

There is partial reflection observed always, even when there is no total internal reflection.
(See pg 73 of textbook)

208
Q

What is total internal reflection?

A

When light going from a more optically dense to a less optically dense material hits the boundary at an angle greater than the critical angle and is completely reflected.

209
Q

What are the conditions for total internal reflection?

A

1) Incident substance has a larger refractive index than the other substance
2) Angle of incidence exceeds the critical angle

210
Q

What is an optical fibre?

A

A very thin flexible tube of glass or plastic fibre that can carry light signals over long distances.

211
Q

Describe how an optical fibre works.

A
  • The fibre has a very high refractive index, but is surrounded by a cladding with a lower refractive index -> This enables TIR + Protects the fibre
  • The fibre is narrow -> Light always hits the boundary at an angle greater than the critical angle
  • Light that enters at one end is totally internally reflected to the other end
212
Q

Name some design features of an optical fibre.

A
  • Thin -> Ensures light hits at angle above the critical angle + Prevents modal dispersion
  • Cladding of lower refractive index -> Protects the fibre from scratches + Ensures TIR happens
213
Q

What are the 2 reasons for cladding on an optical fibre?

A
  • Protects the fibre from scratches

* Ensures TIR happens (by having a lower refractive index)

214
Q

What are the 2 reasons for making an optical fibre thin?

A
  • Ensures light hits at angle above the critical angle

* Prevents modal dispersion

215
Q

Name 2 uses of optical fibres.

A
  • Endoscopes

* Communications

216
Q

What is signal degradation?

A

The disruption and changing of a signal as it passes through an optical fibre.

217
Q

What are the two ways in which a signal can be degraded?

A
  • Absorption

* Dispersion

218
Q

What is signal degradation by absorption and how does it affect the signal?

A
  • Some of the signal’s energy is lost through absorption by the material of the fibre.
  • This reduces the amplitude.
219
Q

What are the two types of signal dispersion?

A
  • Modal dispersion

* Material dispersion

220
Q

What is modal dispersion and how does it affect the signal?

A
  • Light rays enter the fibre at different angles and so take different paths -> Some arrive later than others.
  • This causes pulse broadening.
221
Q

What is material dispersion and how does it affect the signal?

A
  • Different wavelengths of light travel at different speeds in the fibre -> Some arrive later than others.
  • This causes pulse broadening.
222
Q

How can signal degradation by absorption be reduced?

A
  • Use a highly transparent fibre to stop absorption.

* Use an optical fibre repeater

223
Q

How can modal dispersion be reduced?

A
  • Use a very narrow fibre -> Path difference is small.
  • Use a single-mode fibre -> Only allows light to take one path.
  • Use an optical fibre repeater
224
Q

How can material dispersion be reduced?

A
  • Use monochromatic light

* Use an optical fibre repeater

225
Q

What is an optical fibre repeater and what does it prevent?

A
  • A device that boosts and regenerates the signal every so often
  • This reduces signal degradation by absorption and dispersion
226
Q

When signal dispersion is large, what can happen?

A

Broadened pulses can overlap, causing confusion.

227
Q

How does a medical endoscope work?

A
  • Has 2 bundles of fibres.
  • One is used to illuminate the area of the body.
  • A lens forms an image on the end of the other bundle
  • The fibres then take this image back to the other end, where it can be viewed.
228
Q

What is important about the bundle of fibre in an endoscope?

A

The bundle must be coherent, so that the image on the other end is not muddled up.

229
Q

What is a coherent bundle of fibres in an endoscope?

A

When the fibre ends are in the same relative positions (i.e. the fibres arrange themselves in the same order to recreate the original image).

A LEVEL - Physics (14 decks)

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