Unit 2 - Waves Flashcards
(67 cards)
1
Q
Frequency
A
- Number of cycles completed by a wave through a point per second (Hz)
2
Q
Period
A
- Time take to complete a wave (s)
3
Q
Wavelength
A
- Distance from crest to crest (m)
4
Q
Transverse waves
A
- Wave traveling perpendicular to direction of energy transfer
5
Q
Wavefront
A
- The surface made up of all the points of the wave that are in phase with each other
6
Q
Coherent waves
A
- Waves with same: frequency, wavelength, speed and constant phase difference
7
Q
Path difference
A
- Difference of distance between two sources from a fixed point throughout its phase
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
- Can only occur with waves of the same type (electromagnetic with electromagnetic, sound with sound…)
9
Q
Interference
A
- Two waves can superpose resulting in a wave with different amplitudes
- Amplitude obtained by adding displacements (can be negative)
- Amplitude of wave of product of superposition depends on:
· Amplitude of the 2 waves
· Phase relationship
· Where its perceived from
10
Q
Constructive interference
A
- Amplitude is doubled if waves in phase
- Δθ = nλ (phase difference has to be an integer multiple of the wavelength)
11
Q
Destructive interference
A
- Resultant of wave has no amplitude as waves completely out of phase (π)
- Δθ = (λ / 2) · (2n + 1) (phase difference has to be an odd multiple of half the wavelength)
12
Q
Amplitude
A
- Maximum dispplacement from equilibrium
13
Q
Cause of waves
A
- Disturbance at a source causes particles to oscilate about a fixed central point
14
Q
Huygens principle
A
- Every point of a wavefront is a new source of the same kind of wave
15
Q
Diffraction
A
- When a wave passes the edge of an obsatcle, the wave energy spreads into the space behind the obstacle
- It occurs when the size of the aperture or obstacle is of the same order of magnitude as the wavelength of the incident wave
- nλ=dsinθ
16
Q
Intensity of radiaton
A
- Intensity = Power / Area
=> Intensity = Energy / (ΔTime · Area)
=> Intensity = Energy / (ΔTime · 4 · π · r²)
17
Q
Standing wave
A
- Wave appears to be standing still
- Formed when two coherent waves with similar amplitudes travelling in opposite directions are superposed
- Has nodes and antinodes
- No energy transfer
18
Q
Progressive waves
A
- Wave that transfers energy (most types of waves except stading waves)
19
Q
n’th harmonic
A
- n antinodes
- (n + 1) nodes
- λ = 2L / n
20
Q
Node
A
- Zero-displacement point in a standing wave
21
Q
Antinode
A
- Crest / trough in statnding wave
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)
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
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꜀
25
Refractive index
- Relationship of wave speed/wavelength between mediums
- n12 = v1 / v2 = λ1 / λ2
- n >= 1
26
Snell's law
- na · sinθᵢ = nb · sinθᵣ
- 0º < θ < 90º
- Reflection or refraction
27
Total internal reflection
- θᵢ = θᵣ
- θᵢ > θ꜀
- n₁ < n₂
28
θᵢ = θ꜀
- Light travels parallel to boundary
29
Critical angle
- With snell's law, take θᵣ = 90
- sin C = 1 / n
30
Photon
- Fundamental particle
- No mass nor charge, only energy
- Can interact with other charged particles
31
Electromagnetic waves
- Composed by an electric and magnetic field, oscialating perpendicular to eachother
- Travel at c in vaccum
32
Planck's equation
- E = hf
- E = (hc)/λ
33
De Broglie wavelength
- Particles can behave as a wave
- Wavelength given by De Broglie wavelength (inversely proportional to momentum)
- λ = h / p
34
h
- Planck's constant
- 6.63 · 10⁻³⁴ Js
35
Plane polarisation
- Reducing all oscilation planes of a wave to a single one through the use of a polarisation filter
36
Unpolarised waves
- Wave of a mix of all planes (orientation)
37
Plane polarised wave
- Wave with a single oscialtion plane
38
Polarisation filter
- Lets through waves in a single oscilation plane
39
Echolocation
- 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
SONAR
- 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
Ultrasound imaging
- 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
Echolocation pulse duration
- 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
Resolution
- 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
Photoelectric effect
- 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
Work function φ
- 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
Electron Volt (eV)
- 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
Reason emission of photoelectrons is immediate
- 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
Reason EM wave below a threshold frequency can't release electrons
- 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
Reason experiments show KE proportional to frequency but not on intensity
- Photon energy is proportional to frequency by particle theory
- By wave theory, KE is proportional to intensity (which is false)
50
Number of photons inciding
- 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
Planck Einsten relation
- E = hf
- E = hc / λ
- A photon has no mass, only energy
- Energy of photon proportional to frequency of wave
52
Plotting to find h
- Plot KE against frequency
- KE = hf - φ
- Gradient is h
- h · f₀ = φ
- |y-intercept| = φ
- |x-intercept| = f₀
53
Stopping voltage
- Potential difference with which photoelectrons don't have enough energy to be measured
- e · Vs = 1/2 · m · v²
54
Electron diffraction
- 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
Crystallography
- 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
Electron shells
- 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
Absorption and excitation
- 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
Emission
- 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
Energy level diagrams
- 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
Electron energy
- 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
Ionisation
- 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
Neon lights
- High voltage causes ionisation of gas in tube and acceleration of ions
- High-speed collisions provide energy for exitation
- Colour depends on gas
63
Line spectra
- 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
Light missing from reflected beam
- Interference takes place
- Destructive interference occurs when waves meet in antifase
- Path difference = (2n + 1)λ
65
Wavefront
- Line in which all points of wave are in phase
66
Reason light doesn't change direction
- 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
Reason electrons have discrete and maximum levels of energy
- 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
