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Z A Level Physics 2 > Turning points: Wave Particle Duality > Flashcards

Turning points: Wave Particle Duality Flashcards

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1
Q

What did Newton do in 1672?

A
  • Published paper reporting experiments using glass prism to split light into colour spectrum
  • Continuous colour spectrum into red, orange, yellow, green, blue, indigo violet
  • Proposed that colour is a property of light and that white light is a mixture of all different colours
  • Implied that light made up of particles
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2
Q

What did Robert Hooke suggest in 1672?

A
  • Reported on experiments with diffraction

- Suggested on a wave theory of light could account for his results

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3
Q

What did Huygens hypothesise in 1678?

A
  1. He hypothesises that light was a longitudinal wave that propagated through an ‘ether; an invisible medium, which pervaded all space including a vacuum
  2. He produces a geometrical construction that allowed a given future wave front to be located if its present position was known (the wave front is the line or survive on which a wave disturbance has the same phase at all points)
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4
Q

What does Huygen’s principle state?

A
  1. All points on a wave front serve as point sources of spherical secondary wavelets that spread out in the forward direction at the speed of the wave
  2. The new wave front position is the surface that is tangential to all of these secondary wavelets
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5
Q

What could Huygen’s wave theory account for?

A
  • The laws of reflection and refraction

- Also suggested that light slowed down when entering a denser medium

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6
Q

What could his wave theory predict?

A
  1. The wave theory could correctly reduced the diffraction of light through a narrow slot
  2. But did not explain why light formed sharp shadows when passing everyday objects whereas water wave and sound waves clearly diffraction around objects
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7
Q

What did Newton do in 1704?

A
  • Published ‘Optiks’
  • Theory of light in which light is composed of tiny weightless particles that travelled in straight line from the sources
  • Particles called ‘corpuscles’ and his corpuscular theory
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8
Q

What did the corpuscular theory do?

A
  1. Account for the formation of sharp shadows when an opaque object intercepted a beam of sunlight, stating that sharp shadows were formed because the corpuscles that hit the object were stopped
  2. Concluded that corpuscles of light were colours and that different colours moved through a glass prisms at different speeds but could be made to recombine by a second prisms and appear white
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9
Q

How did Fizeau in 1849 determine the speed of light?

A
  1. Shone a light beam at a partially reflecting mirror, which directed the beam between the teeth of a rotating toothed wheel towards a distant mirror several km away
  2. The distant mirror reflected the be a back towards the wheel and the observer
  3. Used a clockwork mechanism with a series of gears to rotate the toothed wheel so that he was able to determine the wheel’s frequency of rotation
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10
Q

What happened when the toothed wheel was stationary?

A

-The light beam passed through a gap in the wheel’s teeth was incident in the distant mirror M and was reflected back, passing through the same gap between the teeth, enabling him to observe continuous light

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11
Q

What happened when he rotated the toothed wheel?

A
  1. The teeth broke up the light beam into pulses and flashes of light were observed
  2. When he continued to increase the frequency of rotation of the wheel, a speed was reached at which the pulse of light leaving through one gap retuned to the wheel at the instant that the next tooth blocked its passes through the wheel
  3. At this speed, he was no longer able to see nay light pulses returning from the mirror
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12
Q

How did he mathematically show the speed of light?

A
  1. Suppose there are N teeth and therefore N gaps equally spaced on the wheel
  2. The time t for the wheel to turn through a distance equal to the width of one tooth is given by t=T/2N
    where T is the time for the wheel to complete one rotation
  3. Since frequency of rotation of the wheel, f is given by f=1/T then t can be written as
    t=1/2Nf
  4. Since the light travels a distance 2D in time t the speed of light c is given by
    c=2D/t
  5. Where D is the distance from the wheel to the distant mirror and therefore:
    c=4DNf
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13
Q

What did Newton propose?

A
  1. Light was formed from corpuscles that were continuously given out in all directions by luminous objects
  2. One consequence was that luminous objects should be constantly losing mass
  3. Could explain reflection, refraction and dispersion
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14
Q

How could the corpuscular theory explain reflection?

A
  • Treated light as solid elastic ball
    1. Change in velocity as ball bounces, and as ball hits surface its vertical component of velocity is reversed but the hormonal component remains unchanged
    2. Ball reflected at same angle at which hit surface
    3. The change in the vertical component of velocity means that the law of reflection can be explained
    4. The angle of incidence is equal to the angle of reflection
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15
Q

How could the corpuscular theory explain refraction?

A
  1. Assumed force of attraction between matter and light
  2. When light corpuscles were in the middle of air or a transparent medium such as glass or water the forces acting on the corpuscles aged on all sides of them and there was no resultant force
  3. But when boundary between air and a medium forces were unbalances and greater force of attraction on the corpuscle
  4. Therefore vertical component of velocity increased
  5. Horizontal component not altered as were equal numbers of air and medium particles on either side so the horizontal forces were still balanced
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16
Q

What did the change in the vertical component explain in refraction?

A
  • Explained why light ray changed direction towards the normal on entering the medium
  • Could also explain the increase in the angle of the light when leaving the medium
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17
Q

What did corpuscular theory require?

A

-The speed of light to be faster in a medium than in air

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18
Q

How could corpuscular theory explain dispersion?

A
  1. Dispersion occurs when different colours of light are refracted by different amounts as they travel through a medium
  2. Newton explain by assuming different coloured light corpuscles had different mass, with red being most massive
  3. So velocities affecting slightly differently when entering the material (red least affected so change direction least)
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19
Q

What was Huygen’s wave theory of light?

A
  • Proposed light was a longitudinal wave similar to sound
  • Longitudinal waves require a medium to travel through
  • Proposed space filled with substance called æther which was transparent and had no inertia (or mass)
  • Could be used to explain reflection and refraction of light
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20
Q

How do waves travel?

A
  • Waves travelled though different material by the propagation of wave fronts
  • Each point on the wavefront acts as a new point source and wavelets spread out from them
  • These wavelets then combined to form a new wavefront
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21
Q

How did Huygen’s wave theory of light explain reflection?

A
  1. When a wavefront was incident on a reflective surface, the point at which the wavefront is reflected acts as a secondary source and new wavelets are formed
    - NOTES
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22
Q

How did Huygen’s wave theory of light explain refraction?

A
  1. As wavefront reaches the boundary it acts as a secondary source of waves
  2. The wavefront is travelling at an angle to the boundary so it takes the wavefront a finite amount of time to pass over the boundary
  3. The wavelets formed as each point of the wave is incident on the boundary spread out in the medium and from a new wavefront inside the material
    - Predicted the speed of light would be slower inside a medium compared with air
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23
Q

What could Newton’s corpuscular theory explain?

A

reflection, refraction, dispersion

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24
Q

What could Newton’s corpuscular theory not explain?

A

diffraction, interference, polarisation

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25
Q

What could Huygen’s wave theory of light explain?

A

reflection, refraction, (dispersion) , diffraction, interference

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26
Q

What could Huygen’s wave theory of light not explain?

A

polarisation

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27
Q

Why could neither theory be confirmed?

A
  • No way to test evidence of æther

- No way to test speed of light at time

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28
Q

What did Young find that the spacing of the fringes depend on?

A
  1. Separation of the double slits
  2. Distance from the double slits to the screen
  3. The colour (wavelength) of the light
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29
Q

What was Huygen’s theory accepted over Newton’s?

A
  • Corpuscular theory could not explain interference pattern

- Corpuscular would go through each slight and only tow bright finger would be seen

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30
Q

What else meant that Huygen’s theory was accepted over Newton’s

A
  • Fresnel and Arago, realised that light was transverse not longitudinal then could explain polarisation
  • Wave theory of light finally widely accepted
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31
Q

What was the set up of Fizeau’s speed of light experiment?

A
  1. Lens L1, light from a source focused onto edge of toothed wheel with 720 teeth
  2. Wheel had regular notches cut into circumference
  3. Light shone through one of the gaps on the edge of the wheel and passed through another lens L2, to form parallel beam of light
  4. Light then travelled into another lens, L3, where it was focused onto a curved mirror, M and reflected back towards L2 and through toothed wheel
  5. Light them observed using eyepiece E
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32
Q

What was the scale of F’s experiment?

A

Two ends of experiment 8km apart and light source travelled over 16km

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33
Q

In what way was Fizeau’s speed of light experiment set up?

A
  1. Light correctly focused and could be seen on eyepiece
  2. Wheel rotated and speed of rotation increased
  3. Intensity of light seen in eyepiece varied
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34
Q

What was the light intensity and maximum or minimum?

A
  • Minimum: when gap had moved and a tooth in its place

- Max: when returning light passed through a gap

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35
Q

What did Fizeau’s experiment show?

A
  1. Light travelled at finite, but very fast speed
  2. Light travels more slowly in materials
    - As wave theory also said light slow down as travel through material supported wave theory
36
Q

What did Maxwell predicted about the speed of the waves?

A

-Waves of oscillating electric and magnetic fields existed which would travel at:
c = 1/ sqrt(epsioln0mu0)
-epsilon0, permittivity of free space and determines strength of electric fields
-mu0 is permeability of free space and determines strength of magnetic fields

37
Q

What did Maxwell’s equations also predict?

A

-Existence of electromagnetic waves other than visible light and ultraviolet

38
Q

How are electromagnetic waves formed?

A
  1. Accelerating charged particles, often electrons
  2. Charged particle produces an electric fields
  3. Moving charged causes a charging electric field perpendicular to the direction of motion
  4. Changing electric field produces a changing magnetic fields perpendicular to electric field and the changing magnetic field produces a changing electric field etc
  5. These oscillations in the magnetic and electric field do not require a medium to propose through and once they begin, will continue to travel through space until interact with matter
39
Q

What is the wavelength of an electromagnetic wave?

A

Shortest distance between two point on the wave where the electric field and magnetic field are in phase

40
Q

What were the two values obtained by F and M?

A
  • F: 313274000ms-1 (experimentation and direct measurement)
  • M: 310740000ms-1 (consequence of of development of math describe for em waves)
  • Two very different methods very similar results
  • Provided support that Maxwell’s equations fro electromagnetism described the observed properties of light (an em wave) and were a correct description of the nature of em waves
41
Q

What did Heinreich Hertz do in 1887?

A

Produce radio waves

42
Q

How did Hertz produce radio waves?

A
  1. Transmitter consisted of a circuit containing a Leiden jar, an infection coil and a loop of wire with small copper spheres on each end
  2. Detector, incomplete loop f copper wire also with small copper spheres at each end
43
Q

What happens when switch closed?

A
  • Indiction coil produces a high voltage across the spark gap
  • This spark (moving charge) caused radio waves to be produced
  • The waves propagated through the air towards the detector
  • These em waves then induced a voltage in the copper wire detector which caused a small spark to jump across the detector gap
44
Q

What happened when a metal plate was placed between the transmitter and detector?

A
  • Prevented waves from reaching the detector
  • Insulators did not appear to affect radio waves
  • Placing a concave shaped metal plate behind the detector made the detector sparks stronger because the radio waves were focused by the plate back onto the detector
45
Q

What were the waves produced by the spark gap transmitter?

A

Polarised

  • If a receiving aerial used to detect waves produced intensity is a maximum when the receiver is parallel to the transmitter spark gap
  • Intensity decreases as aerial rotated through 90 degrees until it is zero when the aerial is parallel to magnetic field of electromagnet field
46
Q

Why were the waves polarised?

A

Motion of the electrons in the spark is in one plane and this sets up electromagnetic waves which have the E field oscillating in one plane only

47
Q

What happens when the em wave reaches the receiving aerial?

A
  • The oscillating electric field causes electrons in the aerial to move and the signal to be detected
  • As angle between ef and aerial is altered, ef causes less motion of electrons in the aerial and the intensity of the signal decreases
  • When the reviving aerial is at right angles to the ed, the aerial will not be affected by the electric field
48
Q

How did Hertz measure speed of radio waves?

A
  1. Created standing waves by reflecting radio waves using a flat metal plate
  2. As moved dipole detector between transmitter and plate, points where signal was max or min and a pattern of antinodes and nodes produced
  3. By measuring distance between two adjacent maximum (antinodes), wavelength and calculated frequency by assuming that generate. of radio waves was due to electrical characteristics of transmitter circuit
49
Q

What was the speed of radio waves?

A
  • Hertz’s calms similar value to predicted by Maxwell

- Shown radio waves type of em radiation

50
Q

What is a black body?

A
  • An object that absorbed or emits all wavelengths of electromagnetic radiation falling on the object and it does not reflect any wavelengths
  • To obtain black bodies, Kirchoff covered objects with back soot from boring lamps
51
Q

What is infrared radiation?

A
  • All objects above absolute zero will emit eelctoramgeiic waves the form of ir radiation
  • Frequency of emitted waves depend on temperature of object and the surface of object
  • Dark matt surfaces absorb and emit radiation between than shiny white
52
Q

Why was Newton’s theory accepted over Huygen’s?

A

Newton viewed as a greater authority

53
Q

What happened as you treat em radiation as a wave?

A
  1. Predicted the energy density of wavelengths given off from a back body would rapidly become infinite and short wavelengths
  2. Measured spectra from back bodies, large increase in energy density at short wavelengths did not happen
  3. The difference between the theoretical values and the experimental values were particularly large at short wavelengths
54
Q

What was the ultraviolet catastrophe?

A

The term used to describe the prediction (made by classical wave theory) that the energy emitted by a black-body would continue to increase as wavelength decreased. This did not describe the behaviour that was observed

55
Q

What did Planck do in 1900?

A
  1. To describe behaviour of back bodies, with constant from experiments and made an assumption that energy was quantised
    E=hf
  2. Meant that theoretical predicted matched experimental results
56
Q

What happens when UV light is shone onto a zinc plate?

A
  1. When UV light shone onto a zinc plate attached to a negatively charged golf leaf electroscope, the electroscope is seen to discharge and shining visible light onto since plate does not cause any change to the electroscope
  2. UV photons have sufficient energy to knock out an electron from the zinc, but the photons of visible light do not have sufficient energy to dislodge an electron from the metal surface
57
Q

What is the photoelectric effect?

A

This occurs when light shining on the surface of a material particularly a metal releases electrons from the material

58
Q

What is intensity?

A

The light energy per second incident on the surface of the metal

59
Q

What features were observed with photoelectricity?

A
  1. No photoelectrons are emitted if the incident light is below a threshold frequency f0, the threshold frequency varies for different metals. Visible light can be used to emit electrons from alkali metal such as sodium and potassium, but other metals require UV light which has a higher frequency
  2. Photoemission stats to occur as soon as light of the appropriate frequency is shone onto the metal surface
  3. As the intensity of incident light increases, the number of photoelectrons emitted also increases
  4. Photoelectrons are emitted with a range of kinetic energies and the maximum kinetic energy depends not the frequency of the incident light as long as it id above threshold frequency
60
Q

What did classical wave theory predict about the photoelectric effect?

A
  • In order to be emitted from the surface of a metal electrons must gain energy
    1. Photoelectrons would be emitted for all frequencies of incident light. The energy transferred could ‘build up’ and so photoelectrons should br emitted for all frequencies of light and there was no threshold frequency
    2. There would be a delay between shining light onto the metal surface and the emission of photoelectrons, At lower frequencies or intensities, it would take a longer time before emission started
    3. The intensity of light should be proportional to the maximum kinetic energy fo the electrons
  • Wave theory not explain observations
61
Q

What did Einstein propose in 1905?

A

-EM radiation made up of discrete quanta of energy rather than a continuous wave so experimental observation could be explained and this follows Planck’s work of bb radiation
-E thought actual physical phenomena photons not just mathematical convince like Planck
-Each photon carries amount of energy E directly proportional to frequency of em radiation
E=hf
-His description of em radiation as being quantised and his explanation for photoelectric effect let to Noble prize and he showed that lighting other em radiation could be thought of as massless particles carrying energy rather than waves

62
Q

How did the photon theory of light explain the experimental observations for the photoelectric effect?

A
  1. In order to leave the surface of a metal, each electron must obtain sufficient energy through an interaction with a single photon and photons with a low frequency do not transfer enough energy and so no electrons are emitted
  2. Electrons need less energy to leave the surface of some metals and the threshold frequency for the metals is lower and the amount of energy required is called the work function of the metal
  3. As the intensity of light increases, the number of photons increases so more electrons can be released from the surface. However each photon still carries the same amount of energy, so if the light is below the threshold frequency, no electrons will be emitted regardless of intensity
  4. The energy transferred by a photon provides the electron with energy to escape the surface of the metal. If the photon energy is greater than this then the additional energy is seen as the kinetic energy of the electron and this means that the maximum kinetic energy will increase with frequency
63
Q

What is needed in order for an electron to be emitted from the surface of a metal?

A

-Gain energy at least equal to the work function of the metal
-If photon catties more energy than the work function, then the electron will gain kinetic energy
hf = phi + KE
-The maximum Ke of the electron will be equal to the energy of the photon less the work function of the metal
KEmax = hf - phi

64
Q

What did de Broglie think?

A
  • All matter particles have a wave associated with them
  • Thought that the wavelength of the matter waves was related to the momentum of the particle
  • =h/lamda
65
Q

Describe electron diffraction

A
  1. Beam of electrons with uniform speed directed through a thin metal foil
  2. When a fluorescent plate is placed not he other side of the foil and developed a pattern of concentric rings is observed
  3. Pattern of diffraction rings similar to those obtained during X ray diffraction experiments
66
Q

What happens in the electron diffraction tube?

A
  • Each plane of atoms in the metal foil behaves like a diffraction grating
  • Causing constructive and destructive interference leading to a pattern of dark and bright rings
67
Q

What happens as the momentum of the electrons increases?

A

-The angle through which they are scattered decreases
-Decrease in wavelength
lamda proportional to sintheta

68
Q

What did de Broglie’s experiment confirm?

A

-Particles could have wave-like properties

69
Q

How do you calculate lamda using electron diffraction pattern?

A

-As electrons accelerated towards anode through a potential difference V, KE equal to work done
1/2mv^2 = ev
lamda = h/mv
lamda= h/ sqrt (2meV)
-Increasing the anode voltage decreases the wavelength fo the matter waves and this gives less scattering

70
Q

What is resolving power?

A

The ability of an optical instrument to distinguish between two features close together on an object under examination. The resolving power is limited by the diffraction of light

71
Q

What does resolving power depend on?

A
  • Related to wavelength of light being used
  • Resolving power depends on how much diffraction of light occurs when the light passes through the objective lens of the microscope
  • A shorter wavelength such as blue light, increases the resolving power and allows finer details not he specimen to be seen
72
Q

Which electrons have shorter wavelengths?

A

-Electrons with sufficient momentum can have much shorter wavelengths than light which means that a microscope using the wave-like properties of electrons will have a much greater resolving power than an optical microscope

73
Q

What is an electron microscope?

A
  • Have lenses, just as optical microscopes
  • Rather than being formed from glass, the ‘lenses’ in an electron microscope alter the path of the electron but he use of a magnetic or electric field
74
Q

What are transmission electron microscope (TEM)?

A
  • The first electron microscopes to be developed were TEM
    1. In TEM a beam of electrons passes through a extremely thin specimen
    2. As the electrons interact with structures within the specimen (e.g. cell walls, grain boundaries in metals) the electrons are scattered
    3. Scattered electrons focused by a number of magnetic lenses and a magnified image of the sample is formed
  • The image may be formed onto photographic film, but other methods of recording the image may be used including CCDs
75
Q

How do you reduce collisions of the electrons with the air molecules in TEM?

A

Inside of TEM reduced to a very low pressure

76
Q

What happens in TEM?

A
  1. The electron gun produces a beam of electrons
  2. The speed and therefore the wavelength of the electrons is controlled using the anode voltage
  3. The condenser lens is used to form the electrons into a parallel beam
  4. This beam then passes through the specimen which is held in place on a specimen stage, (a fine gold wire gird)
  5. The stage can be moved and rotated so that different sections the specimen can be examined
77
Q

What next happens to the scattered beam of electrons?

A
  • Passes through he objective lens which forms a magnified, inverted image of the sample
  • Finally the magnifier lens focuses the electrons from the intermediate image to form a final magnified image not he screen or recording device
78
Q

How can you increase the resolving power of TEM?

A
  • Increase anode pd

- This also enlarges the image on the screen and increases the magnification and detail which can be seen

79
Q

What is the detail that can be seen using TEM limited by?

A
  • The thickness of the specimen used

- By lens aberrations

80
Q

What is crucial of the specimen used in TEM and why?

A
  1. Must be very thin, hundreds of nanometers
  2. As electrons pass through the material, they slow down slightly
  3. This causes their de Broglie wavelength to increase and therefore resolving power to decrease
81
Q

Why does lens aberration occur?

A
  • The scattered electrons will be travelling at slightly different speeds
  • This means that the magnetic lenses may be unable to focus electrons from the same part of the sample onto eh same point on the screen
82
Q

What is a scanning tunnelling microscope (STM)?

A
  1. Uses a very fine tipped probe which moves across, or scans a small area of the surface of a specimen
  2. Has no lens
  3. Makes use of wave-like nature of electrons
83
Q

How does STM work?

A
  1. A probe tip is held very close to the surface of the specimen, typically no more than 1nm above the surface
  2. The top is slightly positive compared with the surface of the specimen
  3. At this small distance electrons can ‘tunnel’ across the gap, causing a tunnelling current to flow
  4. This current increases measurably as the tip gets closer to the surface and decreases if the distance between the tip and surface increase
84
Q

What are the two different modes STM can be used in?

A
  1. Constant current mode
    - The tip is moved across the surface and piezoelectric transducers move the tip up or down as to keep the measured tunnelling current constant
    - The up and down movement can be imaged as a map of the peaks and troughs of the surface
  2. Constant height mode
    - The tip is moved across the surface at a fixed vertical position
    - The change in the tunnelling current can be used to map the height of the surface of the specimen
85
Q

What is characteristic of STM in both modes?

A
  • The resolving power of a scanning electron microscope provides a map of the surface on the scale of individual atoms
  • Vertical resolution 0.1 Å which is smaller than smallest atom
86
Q

Why does the tunnelling current arise?

A
  • The wavelike nature of electrons
  • The prove tip is at a slightly lower energy level because of the positive potential difference applied to it in the STM
87
Q

How does STM used wave-like nature of electrons?

A
  • In classical physics, electrons to possible to cross gap as particles
    1. Amplitude of the wave reduces as it crosses the barrier but does not reduce to zero
    2. Therefore finite probability that the electron can cross to the prove tip causing a tunnelling current
    3. They travel as they disappear from the specimen and immediately reappear in the probe
  • strange quantum behaviour
  • fact that STM can be built and produce images provides evidence that wave-particle duality is a real phenomenon

Z A Level Physics 2 (16 decks)

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