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ALevel Physics NEW > Section 3 - Waves > Flashcards

Section 3 - Waves Flashcards

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1
Q

What is a wave?

A

A periodic disturbance of particles or fields

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2
Q

What is a progressive wave?

A

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

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3
Q

What causes a wave to form?

A

Something making particles or fields oscillate at a source

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4
Q

Define amplitude

A

The maximum magnitude of displacement

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5
Q

Define a cycle of a wave

A

One complete vibration of a wave

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6
Q

Define wavelength

A

Length of one whole wave cycle, crest to crest or trough to trough

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7
Q

Define time period

A

Time taken for a whole cycle to pass a given point

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8
Q

Define frequency

A

No. of cycles passing a point per second

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9
Q

Define phase

A

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

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10
Q

Define phase difference

A

The amount one wave lags behind another

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11
Q

Define reflection

A

The wave bouncing back when it hits a boundary

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12
Q

Define refraction

A

The wave changing direction as it enters a different medium

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13
Q

What is the speed of EM waves in a vacuum?

A

3x10^8 m/s

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14
Q

Define transverse waves

A

Oscillations that are perpendicular to the direction of energy transfer

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15
Q

What type of waves are all EM waves?

A

Transverse

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16
Q

Define longitudinal waves

A

Oscillations that are parallel to the direction of energy transfer

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17
Q

Which direction does a polarised wave oscillate in?

A

Only one direction

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18
Q

What type of waves can be polarised?

A

Only transverse waves

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19
Q

Give evidence that EM waves are transverse

A

They can be polarised

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20
Q

What do polarising filters do?

A

Only transmit vibrations in one direction

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21
Q

What happens if you place 2 polarising filters at right angles to eachother?

A

No light gets through

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22
Q

Describe the graph of the 2 polarising filters, as the second one is rotated from 0 to 360 degrees

A 
0 - Full intensity
90 - 0 intensity
180 - full intensity
270 - no intensity
360 - full intensity
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23
Q

Define partially polarised

A

Some of the wave vibrates in the same direction

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24
Q

Give 2 applications of polarisation

A

Polarised sunglasses, television and radio signals

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25
Q

When does superposition occur?

A

When 2 or more waves pass through each other

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26
Q

Define the principle of superposition

A

When 2 or more waves cross, the resultant displacement equals the vector sum of the individual displacements

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27
Q

What are the 2 types of interference?

A

Constructive and destructive

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28
Q

When does constructive interference occur?

A

When a crest meets a crest, or trough meets a trough

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29
Q

When does destructive interference occur?

A

When a crest meets a trough

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30
Q

What happens when a crest and trough are not the same size?

A

The interference is not total. For it to be noticeable, the amplitudes should be nearly equal

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31
Q

When are 2 points on a wave in phase?

A

When they have the same displacement and velocity

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32
Q

Describe mathematically when 2 points on a wave are in phase

A

They have a phase difference of 0 or 360 deg

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33
Q

Describe mathematically when 2 points on a wave are out of phase

A

They have a phase difference of odd number multiples of 180 deg

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34
Q

How do you get clear interference patterns?

A

By making the 2 sources coherent

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35
Q

Define coherent sources

A

2 sources are coherent if they have the same wavelength and frequency, and a fixed phase difference between them

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36
Q

What does constructive and destructive interference depend on?

A

Path difference

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37
Q

Define path difference

A

The amount by which the path travelled by one wave is longer than the path travelled by the other wave

38
Q

When do you get constructive interference?

A
  • At any point an equal distance apart from 2 coherent and in phase sources
  • When the path difference is a whole number of wavelengths
39
Q

When do you get destructive interference?

A

When the path difference is n+1/2 lamba

40
Q

State the constructive path difference equation

A

Path difference = n lambda

41
Q

State the destructive path difference equation

A

Path difference = (n+1/2)lambda

42
Q

What can be created when a progressive wave is reflected at a boundary?

A

A stationary wave

43
Q

Define stationary wave

A

The superposition of 2 progressive waves with the same frequency and wavelength, and moving in opposite directions

44
Q

How much energy is transmitted by a stationary wave?

A

0

45
Q

What are nodes?

A

When the amplitude of the vibration is 0

46
Q

What are antinodes?

A

When the amplitude of the vibration is the maximum

47
Q

How many wavelengths are there for the first, second and third harmonic?

A

1st - half a wavelength
2nd - 1 wavelength
3rd - 1 1/2 wavelength

48
Q

Describe the 1st harmonic

A
  • Stationary wave with the lowest possible resonant frequency
  • Node at each end with an anti node
  • 1/2 lambda
49
Q

Describe the 2nd harmonic

A
  • Stationary wave with twice the frequency of the first harmonic
  • Node at each end and middle. 2 anti nodes
  • 1 lambda
50
Q

Describe the 3rd harmonic

A
  • Stationary wave with triple the frequency of the first harmonic
  • 4 nodes and 3 antinodes
  • 1 1/2 lambda
51
Q

What experiment can you use to demonstrate stationary waves?

A

Microwaves and sounds

52
Q

Describe how you can demonstrate stationary waves with microwaves

A

Order of equipment left to right:

  • Metal plate, probe connected to loud speaker, microwave transmitter
  • Can find nodes and antinodes by moving probe between transmitter and reflecting plate
53
Q

Describe the experiment you would have to do in order to find the factors affecting the resonant frequency of a string

A
  • Measure the mass, length, and find mu (M/L)
  • Set up apparatus and record mu, measure length and work out tension using T=mg
  • Turn on signal generator until you find first harmonic
  • Explain how length, tension, MPL affect resonant frequency, keep 2 constant and change other (total 3 times)
54
Q

What conclusions should be reached after you have completed the resonant frequency experiment?

A
  • Longer string, lower frequency
  • Heavier the string, lower the resonant frequency
  • Looser the string the lower the resonant frequency
55
Q

What does the amount of diffraction depend on?

A

The wavelength compared to the size of the gap

56
Q

What is the diffraction like when the gap is much bigger than the wavelength?

A

No noticeable diffraction

57
Q

What is the diffraction like when the gap several wavelengths bigger than the wavelength?

A

Noticeable diffraction

58
Q

What is the diffraction like when the gap is the same size as the wavelength?

A

The most it could be

59
Q

What is the diffraction like when the gap smaller than the wavelength?

A

The waves are mostly reflected back, not as much

60
Q

What is monochromatic light?

A

A light with the same wavelength and frequency

61
Q

How can you demonstrate light diffraction with a laser?

A

Have coherent, monochromatic light source and shine it thorough a slit and observe the pattern on the screen

62
Q

What diffraction pattern can you observe in the experiment with a laser and a slit?

A

Central bright fringe, with dark and bright fringes alternating on either side

63
Q

In the diffraction light experiment with a laser, what causes

A

Caused by destructive and constructive interference

64
Q

What does diffracted white light create?

A

A spectra of colours

65
Q

Why does a spectrum get created when white light is diffracted?

A

Because of the different wavelengths in the white light, and they diffract by different amounts

66
Q

What does the intensity of light mean?

A

The number of photons

67
Q

What is intensity?

A

Power per unit area

68
Q

What does the width of the central maximum vary with?

A

Wavelength and slit size

69
Q

What does increasing the slit size do to the diffraction, and what is the effect of this on the central maximum?

A

Decreases the amount of diffraction, leads to a narrower, but more intense central maximum

70
Q

What does increasing the wavelength do to the diffraction, and what is the effect of this on the central maximum?

A

Increases the amount of diffraction, leads to a wider, but less intense central maximum

71
Q

What safety precautions should you take when using lasers?

A
  • Never shine directly at a person
  • Wear safety goggles
  • Avoid shining at a reflective surface
  • Have a warning sign on display
  • Turn the laser off when not needed
72
Q

What does young’s double slit experiment prove?

A

EM radiation’a wave nature

73
Q

In the equation, lambda = wD/s, state what the components are

A 
lambda = wavelength
w = fringe spacing
D = distance from slits to screen
s = slit separation
74
Q

What happens to interference patterns when you diffract light through more than 2 slits?

A

They get sharper - bright bands are brighter and narrower, dark areas are darker

75
Q

In the equation, n lambda = d sin theta, state what the components are

A

d = grating with slits d apart
sin theta = sin of angle between zero order and n order
n = order of line
lambda = wavelength

76
Q

from the n lambda = d sin theta experiment, what conclusions can be drawn?

A
  • If lambda is bigger, theta is bigger-> longer wavelength the more the pattern spreads out
  • d is bigger, sin theta is smaller -> coarser the grating, the less it spreads out
  • for certain n values, sin theta is >0 therefore don’t exist
77
Q

What do diffraction help identify?

A

Elements and calculate atomic spacing

78
Q

What does the refractive index or a material measure?

A

How much it slows down light

79
Q

What happens when light enters a more optically dense medium?

A

It slows down and refracts towards the normal

80
Q

What happens when light enters a less optically dense medium?

A

It speeds up and refracts towards the normal

81
Q

What is an optical fibre?

A

A thin, flexible tube of glass or plastic than can carry light signals

82
Q

What does an optical fibre consist of?

A

A core and cladding

83
Q

What is the job of the cladding in an optic fibre?

A

It allows TIR as the refractive index is lower than the core, and it protects the core from scratches which could lead to signal leakage

84
Q

In terms of signal, what does dispersion and absorption cause?

A

Signal degredation

85
Q

What does absorption do to a wave?

A

Causes a loss in amplitude

86
Q

What does dispersion do to a signal?

A

Cause pulse broadening

87
Q

What is model dispersion?

A

When light rays enter the fibre at different angles and take different paths. Some rays take longer paths than others that go down the middle

88
Q

What is the solution to model dispersion?

A

Use a single mode fibre which only lets light take one path

89
Q

What is material dispersion?

A

Light consists of different wavelengths which travel at different speeds. Some travel and reach the end faster than others

90
Q

What is the solution to material dispersion?

A

Use monochromatic light

91
Q

What are optical fibre repeaters?

A

Something that boosts and regenerates the signal every so often. Can reduce signal degradation caused by absorption and dispersion

ALevel Physics NEW (12 decks)

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