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IAL Physics > Waves > Flashcards

Waves Flashcards

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1
Q

Amplitude

A

The magnitude of the maximum displacement reached by the oscillation in the wave.

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2
Q

Wavelength

A

The distance between one point on a wave and the same point on the next cycle of the wave.

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3
Q

Frequency

A

The number of complete wave cycles that pass a point per second.

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4
Q

Period

A

The time taken for one complete oscillation at one point on a wave.

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5
Q

Wave types:

A
  • Transverse waves
  • Longitudinal waves
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6
Q

Transverse waves

A

The vibration/oscillation of
the wave is perpendicular to the direction of the wave.

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7
Q

Longitudinal Waves

A

The vibration/oscillation of
the wave is parallel to the direction of the wave.

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8
Q

Longitudinal waves:

A
  • Sound, ultrasound, infrasound…
  • Human hearing range is 20Hz - 20kHz
  • Infrasound describes waves with a lower limit of human audibility (generally 20Hz)
  • Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing
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9
Q

Progresive waves

A

Waves which move and transmit energy

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10
Q

Longitudinal waves show:

A
  • Areas of high pressure called compressions
  • Areas of low pressure called rerefactions
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11
Q

Equilibrium

A

A restoring force that brings the particles back toward their equilibrium position

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12
Q

Transverse waves:

A
  • Electromagnetic waves
  • Vibrations on a guitar string
  • Waves on a rope
  • Seismic S-waves
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13
Q

Wavefront (simple)

A

Lines which represent the same point on a wave (e.g. crest)

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14
Q

Phase difference

A

The difference in phase between two points on a wave.

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15
Q

Two points on a wave are in phase…

A

when they are the same point in their wave cycle

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16
Q

Superposition:

A
  • When two waves meet
  • The displacement of the resultant wave is equal to the sum on the individual displacements of the two waves
  • Afterwards, each wave will continue past each other, as the energy progresses in the same direction it was originally travelling.
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17
Q

Superposition of continues waves:

A
  • When the two waves are in-phase, they interfere constructively.
  • When the two waves have opposite-phase, they
    interfere destructively and
    cancel each other out.
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18
Q

Coherence

A

Waves are coherent if they have the same frequency and constant phase difference

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19
Q

Explain how noise cancelling headphones work

A
  • Use the principle of superposition of waves
  • Sound waves detected by a microphone
  • Electronic signal sent to loud speaker to produce an inverted wave
  • Two waves must be 180º out of phase
  • Causing cancellation/ destructive interference
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20
Q

Interference

A

When two coherent sources of continuous waves interact, an
interference pattern is observed.

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21
Q

Path difference

A

The difference in distance travelled by two waves from their sources to the point where they meet.

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22
Q

Constructive interference

A

is a path difference of nλ

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23
Q

Destructive interference

A

is a path difference of (n + ½)λ

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24
Q

Superimposing waves:

A
  • Waves travelling same
    direction we get a travelling wave.
  • Waves travelling opposite
    direction we can get a standing wave ONLY if the waves have the same frequency.
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25
Q

Stationary waves:

A
  • Continuous waves travelling in opposite directions will superimpose continuously, and this can set up a standing wave pattern.
  • The waves need to be COHERENT (of the same speed, frequency,
    similar amplitudes and have a constant phase relationship).
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26
Q

Stationary wave properties:

A
  • The profile of the wave doesn’t move along – it only oscillates.
  • Energy does not pass along a standing waves (it is NOT progressive
    wave).
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27
Q

Progressive Wave

A
  • Energy transferred in one direction.
  • Max amplitudes at all points.
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28
Q

Standing/Stationary Wave

A
  • Energy stored within a fixed system.
  • Max amplitudes at specific points.
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29
Q

Nodes

A

superposition always fully destructive, amplitude is always zero, no vibration

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30
Q

Antinodes

A

points of maximum amplitude

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31
Q

Standing waves form on objects only when…

A

oscillated at resonant frequencies

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32
Q

Lowest frequency possible

A

fundamental frequency or 1st harmonic, f0

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33
Q

Higher frequency stationary waves are called…

A

2nd harmonics and have smaller and smaller wavelengths

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34
Q

string wavespeed

A

√t/u

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35
Q

f =

A

1/λ √t/u

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36
Q

f0 =

A

1/2L √t/u

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37
Q

Intensity of radiation

A

I = P/A

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38
Q

intensity of radiation is proportional to:

A
  • Amplitude squared
  • Frequency squared
39
Q

spherical waves intensity of radiation

A

I = P/ 4π2^2

40
Q

Assuming there’s no absorption of the wave energy, the intensity…

A

It decreases with increasing distance from the source.

41
Q

Diffraction

A

The spreading out of a wave as it passes a gap aperture or passes around an obstacle

42
Q

Maximum diffraction can be achieved if:

A
  • the wavelength of the wave is equal to the size of the gap / obstacle
  • the wavelength of the wave is equal to the size of the object / obstacle
43
Q

wavefront

A

is the set of all locations in a medium where the wave is at the same phase. This could be where all the crests are, where all the troughs are, or any phase in between

44
Q

Huygen’s principle is used…

A

to predict he movement of waves if we know the positions of a wavefront

45
Q

To use Huygen’s principle, we consider:

A
  • That every point on a wavefront is a new source of circular waves travelling forwards from that point
  • After plotting numerous circular waves from the wavefront, we can consider superpositions to determine the new wavefront position
46
Q

The tangents creates (diffraction)…

A

the curve of the new wavefront emerging wither through the gap or around the obstacle

47
Q

A stable interference pattern forms when…

A

overlapping waves are coherent (constant phase difference) with one another

48
Q

Interference patterns:

A
  • Maxima - greatest amplitude - constructuve interface
  • Minima - smallest amplitude - destructive interference
49
Q

Refraction:

A
  • Waves change speed as they cross boundaries between different mediums
  • Wavelength changes during refraction but frequency stays the same
50
Q

change in speed means…

A

change in wavelength

51
Q

speed down –>

A

wavelength down

52
Q

Speed up –>

A

wavelength up

53
Q

Refractive index

A

the ratio of the speed of light in a vacuum to the speed of light in the medium

54
Q

more ligh is refracted if…

A

there is a greater chnage in speed

55
Q

the greater the refractive index –>

A

the greater refraction

56
Q

Shorter wavelength / higher frequency refracted…

A

more strongly, wave speed slowed more

57
Q

Critical angle

A

The angle of incidence (in denser medium) for which the angle of refraction (in less dense medium) is 90º

58
Q

critical angles equation:

A

sin(c) = 1/n

59
Q

Partial reflection

A

Both refraction and reflection occur but not equally

60
Q

Total Internal Reflection:

A
  • When light within a denser medium strikes a boundary with a less dense medium
  • At an angle of incidence that is greater than the critical angle
  • ALL of the light is reflected
61
Q

Uses of Total Internal Reflection:

A
  • Fibre optics
  • Decorative lighting
  • Fibre broadband
  • Medical endoscope
62
Q

Polarisation occurs…

A

when particles are only allowed to oscillate in one of the directions perpendicular to the direction of wave propagation

63
Q

Polarisation cannot occur in…

A

longitudinal waves as they oscillate in the same direction as the direction of motion

64
Q

A transverse wave can be polarised in 2 ways:

A
  • Vertically polarised
  • Horizontally polarised
65
Q

Unpolarised Light:

A
  • The oscillations of the electric/magnetic fields of an electromagnetic waves occur in all directions
  • osciallte perpendicular to the direction of energy transfer
66
Q

Polarised light:

A
  • The oscillations of the electric / magnetic fields of an electromagnetic waves occur in only one plane
  • Osciallate perpendicular to the direction of energy transfer
67
Q

Partially-polarised waves:

A

Most oscillations near a single plane, e.g. reflections from surfaces

68
Q

Light can be polarised by…

A

making them pass through a polarising filter (also known as a polariser)

69
Q

A polariser with a vertical transmission axis…

A

only allows vertical oscillations to be transmitted through the filter

70
Q

ligh is said to be partially polarised light if…

A

the intensity of light varies between maximum and minimum for every rotation of 90º of the analyser

71
Q

Diffraction grating

A

is a large number of slits equally spaced. It will cause multiple diffraction patterns that superpose.

72
Q

diffraction gratting equation

A

s = 1 / N

73
Q

Electron diffraction:

A
  • The electrons are accelerated in an electron gun to a high potential and then directed through a thin film of graphite
  • Graphite film is ideal for this purpose because of its crystalline structure
  • The diffraction pattern is observed on the screen as a series of concentric rings
74
Q

larger accelerating voltage

A

reduces the diameter of a given ring

75
Q

lower accelerating voltage

A

increases the diameter

76
Q

The da Broglie Relation

A
  • De Broglie theorised that not only do EM waves sometimes behave as particles, but that very small, fast-moving particles like electrons could also behave as
    waves.
    -
77
Q

The de Broglie hypothesis states…

A

that all particles have a wave nature and a particle nature, and that the wavelength of any particle can be found using the following equation:

77
Q

The greater the momentum… (da Broglie)

A

the smaller the de Broglie wavelength

77
Q

What was the effect of changing the accelerating voltage of electrons on
the electron diffraction pattern?

A
  • Higher energy and momentum result in a shorter de Broglie wavelength, allowing electrons to probe structures on a smaller scale (increases resolution)
  • With a shorter wavelength, the waves are diffracted less and so the
    diameters of the diffraction rings decrease.
78
Q

Two-slit electron interference

A
  • An interference pattern is built up by the movement through the
    apparatus of the individual electrons.
  • Electrons behave as both individual particles and waves at the same time.
  • Wave-particle duality!
78
Q

Electron microscopy:

A
  • Electrons’ de Broglie wavelength is shorter than the wavelength of light microscopes
  • It can be made ever shorter by increasing speed and hence momentum
  • The shorter the wavelength, the better the resolution in microscopes.
78
Q

Electron microscopy

A

The shorter the wavelength, the better the resolution in microscopes.

78
Q

The photon model

A
  • Photons are fundamental particles which make up all forms of electromagnetic radiation
  • The higher the frequency of EM radiation, the higher the energy of the photon
  • The energy of a photon is inversly proportional to the wavelength
  • A long-wavelength photon of light has a lower energy than a shorter-wavelength proton
78
Q

Electron structure:

A
  • Electrons in an atom orbit around the nucleus at particular distances, known
    as energy levels
  • A certain number of electrons can occupy each energy level
  • The higher the energy level, the further the distance of the electron from the nucleus
79
Q

Atomic Line Spectra

A
  • Electrons cannot stay in a continuous state of excitation, so they will move
    back to lower energy levels through de-excitation
  • An emission line spectrum is produced when an excited electron in an atom moves from a higher to a lower energy level and emits a photon with an energy corresponding to the difference between these energy levels
  • During de-excitation, energy must be conserved, so transitions result in an emission of photons with discrete frequencies
  • Since there are many possible electron transitions for each atom, there are many different radiated wavelengths
  • This creates a line spectrum consisting of a series of bright lines against a dark background
  • An emission line spectrum acts as a fingerprint of the element
  • Each element produces a unique emission line spectrum due to its unique set of energy levels.
80
Q

line of the emission spectrum

A
  • Each line of the emission spectrum corresponds to a different energy level transition within the atom
  • Electrons can transition between energy levels absorbing or emitting a discrete amount of energy
  • An excited electron can transition down to the next energy level or move to a further level closer to the ground state
81
Q

Energy required to move from one energy level to another…

A

is given by the difference of energy between the two energy levels

82
Q

A negative value for the energy implies…

A

that energy must be supplied to the system if the electron is to overcome the attractive force of the nucleus and escape from the atom.

83
Q

The electron volt

A

The electronvolt is a unit of energy, susally used to express small energies

84
Q

1eV is equal to…

A

The kinetic energy of an electron accelerated across a potential difference of 1V

85
Q

Joules to eV

A

divide by 1.6 x10^-19

86
Q

eV to joules

A

multiply by 1.6 x10^-19

87
Q

Photoelectricity

A
  • Where photoelectrons are emitted from the surface of a metal after light about
    a certain frequency is shone on it.
  • Main evidence that light acts as a particle (photons)
88
Q
A

IAL Physics (5 decks)

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