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Physics A Level > Unit 2 - Waves > Flashcards

Unit 2 - Waves Flashcards

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1
Q

Frequency

A
  • Number of cycles completed by a wave through a point per second (Hz)
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2
Q

Period

A
  • Time take to complete a wave (s)
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3
Q

Wavelength

A
  • Distance from crest to crest (m)
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4
Q

Transverse waves

A
  • Wave traveling perpendicular to direction of energy transfer
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5
Q

Wavefront

A
  • The surface made up of all the points of the wave that are in phase with each other
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6
Q

Coherent waves

A
  • Waves with same: frequency, wavelength, speed and constant phase difference
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7
Q

Path difference

A
  • Difference of distance between two sources from a fixed point throughout its phase
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8
Q

Wave superposition

A
  • If two waves interact, a new temporal wave is formed
  • After, the two initial waves carry on the exact same properties as before
  • Waves of same type
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9
Q

Interference

A
  • Amplitude of wave of product of superposition depends on:
    · Amplitude of 2 waves
    · Phase relationship
    · Where its perceived from
    · A = A₁ + A2₂
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10
Q

Constructive interference

A
  • Amplitude is doubled if waves in phase
  • Δθ = nλ
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11
Q

Destructive interference

A
  • Resultant of wave has no amplitude as waves completely out of phase (π)
  • Δθ = (λ / 2) · (2n + 1)
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12
Q

Amplitude

A
  • Maximum dispplacement from equilibrium
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13
Q

Cause of waves

A
  • Disturbance at a source causes particles to oscilate about a fixed central point
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14
Q

Huygens principle

A
  • Every point of a wavefront is a new source of the same kind of wave
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15
Q

Diffraction

A
  • When a wave passes the edge of an obsatcle, the wave energy spreads into the space behind the obstacle
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16
Q

Intensity of radiaton

A
  • Intensity = Power / Area
    => Intensity = Energy / (ΔTime · Area)
    => Intensity = Energy / (ΔTime · 4 · π · r²)
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17
Q

Standing wave

A
  • Wave appears to be standing still
  • Formed when two progressive waves are superposed
  • Has nodes and antinodes
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18
Q

Progressive waves

A
  • Same frequency
  • ~Same amplitude
  • Same speed
  • Same wavelength
  • Constant phase relationship
  • Travelling in opposite directions
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19
Q

n’th harmonic

A
  • n antinodes
  • (n + 1) nodes
  • λ = 2L / n
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20
Q

Node

A
  • Zero-displacement point in a standing wave
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21
Q

Antinode

A
  • Crest / trough in statnding wave
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22
Q

Velocity energy transfer of standing wave

A
  • velocity = √(applied tension / linear mass density)
  • linear mass density = mass / string length
    => frequency = λ⁻¹ · √(T / μ)
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23
Q

Longitudonal vs Stationary

A
  • Same frequency
  • Different wavelength
  • Sound waves transfer energy, stationary dont
  • Sound waves longitudinal, stationary transverse
  • Sound waves same amplitude for all points, stationary waves dont
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24
Q

Refraction

A
  • Change of wave speed as it crosses boundaries between different mediums
  • Wavelength changes, frequency stays the same
  • Angle measured from normal
  • Slowing down = smaller wavelength
  • Speeding up = larger wavelength
  • The higher the frequency, the more it refracts
  • θᵢ < θ꜀
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25
Q

Refractive index

A
  • Relationship between speed of wave between mediums
  • n = c / v
  • n = λ / λᵢ
  • n >= 1
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26
Q

Snell’s law

A
  • na · sinθᵢ = nb · sinθᵣ
  • 0º < θ < 90º
  • Reflection or refraction
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27
Q

Total internal reflection

A
  • θᵢ = θᵣ
  • θᵢ > θ꜀
  • n₁ < n₂
28
Q

θᵢ = θ꜀

A
  • Light travels parallel to boundary
29
Q

Critical angle

A
  • With snell’s law, take θᵣ = 90
  • sin C = 1 / n
30
Q

Photon

A
  • Fundamental particle
  • No mass nor charge, only energy
  • Can interact with other charged particles
31
Q

Electromagnetic waves

A
  • Composed by an electric and magnetic field, oscialating perpendicular to eachother
  • Travel at c in vaccum
32
Q

Planck’s equation

A
  • E = hf
  • E = (hc)/λ
33
Q

De Broglie wavelength

A
  • λ = h / p
34
Q

h

A
  • Planck’s constant
  • 6.63 · 10⁻³⁴ Js
35
Q

Plane polarisation

A
  • Reducing all oscilation planes of a wave to a single one through the use of a polarisation filter
36
Q

Unpolarised waves

A
  • Wave of a mix of all planes
37
Q

Plane polarised wave

A
  • Wave with a single oscialtion plane
38
Q

Polarisation filter

A
  • Lets through waves in a single oscilation plane
39
Q

Echolocation

A
  • Use of sound waves to locate objects
  • Distance travelled by wave is twice to that of the distance from the object to the emitter
40
Q

SONAR

A
  • High frequency sound waves are emitted wich bounce back and are detected
  • Measuring time and intensity, location, size and shape of underwater objects can be detected
41
Q

Ultrasound imaging

A
  • Transducer emits ultrasound waves
  • Waves reflected from each boundary due to density change
  • Transducer receives multiple echoes at different times
  • Distances can be calculated
42
Q

Echolocation pulse duration

A
  • Waves emitted in pulses separated by periods of silence
  • Reasons:
    · Transducers can’t emit and detect simultaneously
    · If incoming and outgoing pulses overlap, information is lost
43
Q

Resolution

A
  • Ability to distinguish between closesly spaced objects
  • Shorter wavelengths produce better resolutions as they diffract less so they interfere less with their reflection
  • If wavelength same as object, it diffracts the most
44
Q

Photoelectric effect

A
  • When EM wave of certain wavelenth/frequency hits metal, its energy (photon energy) is transferred to electrons and are released as photoelectrons
  • Can only explained when considering EM wave as a particle
  • A single photon transfers energy to a single electrons
  • Only released if frequency above threshold frequency (f₀)
  • Increasing intensity of light (photons / second) increases number of electrons released
  • Energy of wave concentrated into a photon which transfers all its energy instantaneously
45
Q

Work function φ

A
  • Minimum energy required to free an electron with no speed
  • If incident photon has larger energy than work function, electron may be released with a speed (kinetic energy)
  • hf = φ + Tₑ
  • hf = φ + 1/2 · m · v²
  • Depends on position of electron in metal
  • Electron on the surface only φ needed
  • The deeper and close to a positive ion, the more energy is needed
46
Q

Electron Volt (eV)

A
  • Unit of energy equal to the work done on an electron required to accelerate it through a potential difference of one volt in a vaccum
  • 1 eV = 1.6 · 10⁻¹⁹
47
Q

Reason emission of photoelectrons is immediate

A
  • One photon interacts with one electrons and releases one electron by particle theory
  • Wave theory allows energy to build up so won’t be instantaneous
48
Q

Reason EM wave below a threshold frequency can’t release electrons

A
  • Frequency too low as not enough energy is released by photon to release an electron (E = hf)
  • By wave theory, even if low frequency, given enough time it will release as energy can build up
49
Q

Reason experiments show KE proportional to frequency but not on intensity

A
  • Photon energy is proportional to frequency by particle theory
  • By wave theory, KE is proportional to intensity (which is false)
50
Q

Number of photons inciding

A
  • Total number: Total source energy transmitted / Energy of photon
  • Amount per second: Source power / Energy of photon
  • Increase number of photons each second by increasing intensity
51
Q

Planck Einsten relation

A
  • E = hf
  • E = hc / λ
  • A photon has no mass, only energy
  • Energy of photon proportional to frequency of wave
52
Q

Plotting to find h

A
  • Plot KE against frequency
  • KE = hf - φ
  • Gradient is h
  • h · f₀ = φ
  • |y-intercept| = φ
  • |x-intercept| = f₀
53
Q

Stopping voltage

A
  • Potential difference with which photoelectrons don’t have enough energy to be measured
  • e · Vs = 1/2 · m · v²
54
Q

Electron diffraction

A
  • Gap between carbon atoms in graphite is similar to electron wavelength
  • Electron beams diffract through graphite, forming interference patterns
  • Graphite acts like a diffraction grating
55
Q

Crystallography

A
  • Determine structure of large molecules
    1. Freeze molecule in stable crystal
    2. Fire photons at different angles with high frequency (X-rays: λ = gap between atoms)
    3. Photons diffract causing patterns
    4. Determine structure with computer analysis
56
Q

Electron shells

A
  • Electrons organised in shells with increasing number of electrons
  • The further away from the nucleus, the larger the energy
  • Ground state: Default/Normal state of electron
57
Q

Absorption and excitation

A
  • Electrons can gain energy from EM waves or collisions
  • Gained energy may cause them to jump up a shell
  • Absorbed energy must match exactly shell energy difference
58
Q

Emission

A
  • Exited electrons are unstable and de-exite to lower orbits by losing energy
  • When de-exiting, a photon is emitted with energy (frequency - hf) equal to difference in shells
59
Q

Energy level diagrams

A
  • Energy level diagrams show the energies associated with each electron shell - Energy represented by a horizonal line
  • Lowest energy level (n=1) shown at the bottom of the diagram
60
Q

Electron energy

A
  • Negative potential energy
  • Maximum energy = 0J (free electron)
  • Gets more negative as shell decreases
  • Energy represents how much work must be done to free electron
61
Q

Ionisation

A
  • If atom absorbs enough energy an electron leaves the atom
  • Atom becomes positve ion
  • The ionisation energy is equal to the energy of the ground state which would free an electron
62
Q

Neon lights

A
  • High voltage causes ionisation of gas in tube and acceleration of ions
  • High-speed collisions provide energy for exitation
  • Colour depends on gas
63
Q

Line spectra

A
  • If gas is heated photons are emitted in a characteristic pattern dependant on electron shells
  • If light emitted is difracted, spectra is produced
  • The more shells, the more possible photon energies, the more possible lines
64
Q

Light missing from reflected beam

A
  • Interference takes place
  • Destructive interference occurs when waves meet in antifase
  • Path difference = (2n + 1)λ
65
Q

Wavefront

A
  • Line in which all points of wave are in phase
66
Q

Reason light doesn’t change direction

A
  • Wavefronts parallel to the boundary
  • Wavefronts enter glass at the same time
  • Wave slows dow
  • As whole wavefront travels the same distance in the same time ray doesn’t change direction
67
Q

Reason electrons have discrete and maximum levels of energy

A
  • Waves are continuos energy source that allow energy build up
  • No maximum energy and would only increase
  • One photon interacts with one elctrons
  • Each photon has a discrete level of energy so each released electron will have a discrete energy level

Physics A Level (5 decks)

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