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Useful Physics Stuff > Wave-Particle Duality + Special Relativity > Flashcards

Wave-Particle Duality + Special Relativity Flashcards

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1
Q

When was Newton’s corpuscular theory?

A

1670s

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2
Q

When was Huygens’ wave theorgy?

A

1690s

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3
Q

What were the details of Newton’s theory?

A

Light is made of ‘corpuscles’ - particles which are perfectly elastic, rigid, weightless, and travel in straight lines

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4
Q

What were the details of Huygens’ theory?

A

Light is made of waves, with wave fronts made from secondary wavelets.

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5
Q

Why was Newton’s theory initially preferred?

A

He was the more famous scientist.

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6
Q

How did Newton’s theory address reflection?

A

Corpuscles would bounce off surfaces

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7
Q

How did Huygens’ theory address reflection?

A

Wavelets would be reflected

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8
Q

How did Newton’s theory address refraction?

A

Corpuscles attracted to boundary, travel faster in a denser medium (wrong!!!)

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9
Q

How did Huygens’ theory address refraction?

A

Waves travel slower in denser media (correct!!)

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10
Q

What could Huygens’ theory not explain?

A

Nothing!

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11
Q

What could Newton’s theory not explain?

A

Diffraction, interference, polarisation

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12
Q

When was Young’s double slit experiment?

A

1801

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13
Q

When was wave theory accepted, and why?

A

1850s+, corpuscular theory could not explain polarisation.

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14
Q

What happened in Young’s double slit experiment?

A

Light going through 2 slits formed a pattern of fringes - waves could make this by superposing either constructively or destructively, but is not possible to explain with the corpuscular theory.

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15
Q

What is a black body?

A

A body that emits all wavelengths of radiation possible at a given temperature.

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16
Q

What is the ultraviolet catastrophe?

A

Prediction where most light would be emitted at short wavelengths - exponentially increasing at ultraviolet. Doesn’t happen.

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17
Q

What law explains the absence of the ultraviolet catastrophe?

A

Planck’s interpretation of waves, where E = hf and travel in quanta.

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18
Q

What did Planck suggest?

A

All radiation is emitted or absorbed in quanta (photons)
Photons are not corpuscles
Energy of the quantum depends on frequency of the radiation, E = hf
No ultraviolet catastrophe as higher frequency photons carry more energy

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19
Q

How did Einstein explain the observations of the photoelectric effect?

A

Using photons - needing both wave and particle explanations depending on what phenomenon was being observed

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20
Q

What photoelectric observation was explained by wave theory? (ignore)

A

More intense radiation leads to more photoelectrons emitted per second - as more energy ‘accumulated’ by metal, so more electrons.

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21
Q

What photoelectric observation could be explained by quanta and wave theory - quanta explanation.

A

More intense rad = more photoelectrons per second - more photons absorbed by metal, so more electrons.

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22
Q

What was the explanation for the photoelectric threshold frequency?

A

Minimum frequency = minimum energy of PHOTON = work function, could not be a wave as intensity had no effect on work func.

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23
Q

What was the explanation for the instantaneous emission of electrons in the photoelectric effect?

A

Using quanta, electrons emitted soon as photon absorbed - not a wave as that would take time to deposit energy.

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24
Q

What was the explanation for the range of kinetic energies present in the photoelectric effect?

A

Range of kinetic energies up to a maximum, using quanta Ek = hf - work func.

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25
Q

How can you find work function from stopping potential?

A

Work function is the intercept of stopping potential against frequency. Threshold frequency is when stopping pot. = 0

26
Q

How does a high intensity affect photocurrent?

A

Increases it, as a higher intensity wave means more photoelectrons per second.

27
Q

What is the setup to measure stopping potential?

A

Vacuum with a window to transmit UV light into it, and an anode and cathode. Then by using a variable power supply you can find stopping potential at a certain frequency of light, by changing v until no light makes it through the vacuum.

28
Q

What was the speed of light thought to have been before Fizeau’s experiments?

A

Infinite

29
Q

What was Fizeau’s experiment?

A

A toothed wheel with n teeth at a distance d between mirrors. Wheel span at frequency f.

30
Q

What did Fizeau find?

A

That when spinning his wheel at frequency f light JUST got through the gap, so c = 4dnf.

31
Q

What are the two postulates of Einstein’s theory of special relativity?

A

Physical laws have the same form in all inertial frames
The speed of light in free space is invariant (shown by Michelson-Morley)

32
Q

What is an inertial frame of reference?

A

A reference frame moving at a constant velocity (i.e no acceleration)

33
Q

What are the equations relevant to wave-particle duality?

A

wavelength = h/p = h/(meV)^0.5

34
Q

What are de Broglie’s equations?

A

mv x lambda = h

35
Q

What does increasing V do for low-energy diffraction experiments? (voltage)

A

Increases energy and so speed of the electrons
Decreases the de broglie wavelength
Produces rings in a diffraction pattern which are brighter and have a smaller radius

36
Q

What was the Michelson-Morley experiment designed to do?

A

Measure the impact of the aether, a theoretical medium for the transmission of light. Did not exist, returned a null value.

37
Q

What was the prediction of the Michelson-Morley expeiment?

A

That light would move more slowly when Earth is moving ‘against’ the aether than with it. Would manifest as a shift in an interference pattern.

38
Q

What did the Michelson-Morley experiment prove?

A

Light is not affected by the earth’s motion, so the speed of light is constant.

39
Q

What was the setup for the Michelson-Morley experiment?

A

Monochromatic light source was split into two by a semi-silvered glass block. Then moves away and recombines - if any slowing was present in one direction it would manifest as an interference pattern.

40
Q

Why do electron microscopes have such a high resolving power?

A

Because the wavelength of an electron is much less than the wavelength of visible light.

41
Q

What is the mode of operation for a TEM?

A

Beam of electrons passes through a sample, resolution about 10^-10m

42
Q

What is the mode of operation for a STM?

A

Fine-tipped probe scans the surface, resolution about 10^-12m

43
Q

Where is an image formed on a TEM?

A

Fluorescent screen

44
Q

How is an image formed on an STM?

A

Using the P.D produced when the height is kept constant - or using the height over time when the P.D is constant.

45
Q

What are TEMs focused with?

A

Magnetic lenses, deflection depends on speed

46
Q

What is the detail of a TEM limited by?

A

Lens aberration, slight variations in the speed of electrons.

47
Q

What is the detail of an STM limited by?

A

The resolution of measuring instruments

48
Q

What did Hertz use to discover radio waves?

A

High voltage sparks jumping across air to produce radio waves. Detected using a loop of wire with a gap, detecting mag field as it causes P.D changes and so a spark across the gap.

49
Q

What did Hertz find about radio waves?

A

The speed by using the wavelength;
The frequency, and that c = f theta
That they were polarised by a change in signal strength when he rotated the dipole.

50
Q

What did Maxwell predict?

A

The existence of self-propagating waves of oscillating magnetic and electric fields (EM waves). Predicted speed too, validated by Fizeau.

51
Q

What is the equation for time dilation, and what do the symbols represent?

A

t = t(0) / root(1-(v^2/c^2)). t0 is proper time, stationary with respect to the clock. T is the time measured by an observer with speed V relative to the clock - time moves more slowly for them.

52
Q

What is the equation for length contraction, and what do the symbols represent?

A

l = l0(root(1-(v^2/c^2)). L0 is proper length (stationary with respect to the clock). L is the length measured by observer with speed v relative to the clock.

53
Q

What is the equation for mass and energy in special relativity?

A

E = m0c^2 / root(1-(v^2/c^2)), kinetic energy. At zero speed E = moc^2, rest energy. At speed v, Ek = mc^2 - moc^2

54
Q

What is the equation for mass in special relativity?

A

m = m0 / (1-(v^2/c^2)).

55
Q

What is the evidence for time dilation and length contraction?

A

Muon decay - more muons went further through the atmosphere than would be expected given their half-live; turns out they were moving close to the speed of light so time dilated and length contracted so they went further.

56
Q

What is the evidence for mass and energy under special relativity?

A

Bertozzi measured k.e of electrons at different speeds, getting results within 10% of predicted values.

57
Q

How do mass and energy increase as the speed approaches c?

A

Exponentially, so is impossible to actually reach c.

58
Q

What parts of the photoelectric effect couldn’t be explained by wave theory?

A

Threshold frequency doesn’t work as wave theory suggests any freq = emission
Immediate emission contradicts wave theory
Increasing intensity does not increase speed as predicted by wave, but increases per sec.
Photoelectrons have range of k.e

59
Q

How did Einstein explain the Photoelectric effect?

A

Photons - packets with particley interactions. All energy transferred when a photon interacts with an electron. Explains WF.
Energy instantly transferred.
Intensity = photons per second. So more photons = more interactions/s.
All electrons get same amount of energy, yet deeper in metal will lose due to collisions == range of k.e

60
Q

What was the setup of the Michelson-Morley experiment?

A

2 beams of light perp. to each other forming an interference pattern - idea is one would be slowed by aether so rotating apparatus would change interference pattern.

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