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Physics 1B > Quantum > Flashcards

Quantum Flashcards

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1
Q

What prediction did classical physics make about the emittance of a body?

A

an ultraviolet catastrophe

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2
Q

What is a photon?

A
  • a packet or ‘chunk’ of radiation energy - NOT a particle

- a probability wave

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3
Q

Equations for the photoelectric effect?

A 
E{photon} = W + KE = W + eV
W = hf where f = threshold frequency at V = 0 
KE{max} = eV{o} where V{o} = stopping voltage
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4
Q

What predictions were made of the photoelectric effect?

What observations were made?

A

Predicted (classical physics) as intensity increases:

  • more electrons emitted
  • KE of electrons increases, so higher stopping voltage
  • no dependence on frequency
  • there will be a time delay

Observed as intensity increased:

  • more electrons emitted
  • no change to stopping voltage > KE did not increase
  • no time delay
  • dependence on frequency
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5
Q

What is Einstein’s postulate?

A

energy in the field itself is quantised into amounts hf as if made up of particles – photons

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6
Q

What are two types of X-ray produced and how are they produced?

A
  • Brehmsstrahlung radiation (‘braking’ radiation): electron loses energy when interacting with nuclei, slows down and emits a photon
  • Characteristic radiation: incoming electron strikes subshell electron, the subshell electron gains energy, and when it de-excites, releases a photon of characteristic energy
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7
Q

What does a X-ray diagram look like? (incl. axes & labels, cut-off points)

A
  • y-axis: X-ray intensity
  • x-axis: wavelength
  • brehmsstrahlung curve
  • some characteristic peaks
  • cut-off point minimum wavelength/maximum photoenergy
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8
Q

What fact and equation did the Compton effect prove?

A

That photons have momentum, p = h/λ{de broglie}

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9
Q

What was puzzling about the results of the Compton experiment?

A

At different angles, a second peak appeared: the constant peak was caused by strongly-bound electrons not causing a scattering effect; the second peak caused by a difference in wavelength

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10
Q

What is the equation for the Compton effect?

A

λ{f} - λ{i} = h/mc * (1 - cosθ), where θ = angle of scattering

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11
Q

When is the Compton effect greatest?

A

When θ = 180 deg / π rad

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12
Q

What is the de Broglie wavelength?

A

The natural wavelength of all quantum particles

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13
Q

What experiment confirmed de Broglie’s theory on the wave nature of quantum particles?

A

Bragg Scattering, showed electron diffraction

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14
Q

What is the equation for the Bragg Scattering? Where does the maximum occur?

A

2dsinθ = n*λ, maximum at 2d = λ

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15
Q

What is the Heisenberg Uncertainty Principle?

A

Position and momentum are ‘incompatible observables’: Δp * Δx > [h bar]/2

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16
Q

The square of the absolute value of the wavefunction is proportional to…

A

… the probability density of finding a ‘particle’ in a small region around position x.

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17
Q

What does the Schrodinger Equation mean?

A

It shows that the probability of finding a particle in some dx is ABSOLUTELY UNIFORM. At any time, until a measurement is made, the position of the particle is entirely unknown.

18
Q

When a measurement is taken, quantum probability becomes certainty. This is known as…

A

Quantum decoherence.

19
Q

What are the Bohr postulates?

A
  • electrons in an atom move in circular orbit, and the angular momentum l is quantized: l = mvr = n*[h bar]
  • an electron in one of these orbits is stable, but if it
    discontinuously changes its orbit, energy is emitted or
    absorbed in photons satisfying: ΔE = hf = [h bar]*ω
20
Q

What are the values for the Bohr radius and 1 Rydberg?

A

Bohr radius a{o} = 0.0529 nm

1 Rydberg = 13.6 eV

21
Q

What are the formulae for r and E in Hydrogen-like atoms?

A 
r = (n^2 / Z) a{o}
E = - (Z/n)^2 Ry
22
Q

State the successes and failures of the Bohr model of the atom.

A 
Successes:
- explained stable atoms
- quantised energy levels
- emission/absorption spectra of H 
Drawback:
- fails in detail for atoms other than H 
- l = 0 was omitted
23
Q

How do we improve the Bohr atom model?

A
  • reduced mass = mM / (m + M)

- altered Rydberg constant: R/R{infinity} = 1 / (m/M +1)

24
Q

Which correction included the “fine structure” of atoms and atomic orbitals?

A

Bohr-Sommerfeld correction

25
Q

How many different orientations can a photon have and what are they?

A

Four: Horizontal, Vertical, +/- 45 deg

26
Q

+ and x photon orientations are…

A

… incompatible observables.

27
Q

Measurements in quantum are…

A

… probabilistic, instead of deterministic.

28
Q

In quantum cryptography, if a message is read by a hacker, what proportion of the signal that is returned to the original recipient carries errors?

A

1/4th of the message is erroneous.

29
Q

What crystal is used to polarise light?

A

Birefringent crystals.

30
Q

What is the difference between the classical and quantum polarisations of 45 degree-oriented light through a H/V crystal?

A
  • classically, 50% of the light will take a H-path, and the other 50% will take the V-path
  • in quantum measurements, this becomes a probability: 50% probability of photon taking H-path, and 50% probability of photon taking V-path.
31
Q

A photon in an H or V state, going through a H/V crystal, is in an [blank] of the measurement operator.

A

Eigenstate.

32
Q

What is the name of the operator that changes the polarisation of light?

A

Pockels cell.

33
Q

What causes the quantisation of energy?

A

The confinement of waves (matter or probability waves) in a potential well.

34
Q

What is the expression that defines the length of any potential well, or the waves confined in it?

A

Each potential well must have a length of integer 0.5 de Broglie wavelengths. L = n * 0.5 * λ{dB}

35
Q

What is the equation of a wavefunction in an infinite potential well?

A

Ψ = sqrt(2/L) * sin(nπx/L)

36
Q

What is the relativistic kinetic energy of a particle?

A

KE = sqrt(p^2c^2 + m^2c^4)

37
Q

True or false: the probability equation for finding a quantum particle in an infinite well has an oscillating part only.

A

False.

38
Q

True or false: superposition of wavefunctions works on the same principle as classical waves.

A

False.

39
Q

What are the two possible approaches to quantum computing?

A

Superposition of wavefunctions, and quantum entanglement.

40
Q

If two wavefunctions interfere destructively, what is the probability compared to the classical probability?

A

If from the wavefunction p(x) < p{cl}, the probability amplitudes interfere destructively.

Physics 1B (3 decks)

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