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A LEVEL - Physics > Section 2 - EM Radiation and Quantum Phenomena > Flashcards

Section 2 - EM Radiation and Quantum Phenomena Flashcards

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1
Q

What is the photoelectric effect?

A

Shining a light of high enough frequency onto the surface of a metal causes it to emit electrons.

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2
Q

What is the frequency required for the photoelectric effect in most metals?

A

In the UV range.

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3
Q

Describe how the photoelectric effect works.

A
  • Free electrons on the surface absorb energy from photons.
  • If an electron absorbs enough energy, the bonds holding it to the metal break and it is released.
  • The emitted electrons are called photoelectrons.
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4
Q

What 3 conclusions can be made from experiments on the photoelectric effect?

A
  1. For a certain metal, no photoelectrons are emitted if the radiation is below a threshold frequency.
  2. Photoelectrons have varying kinetic energies, up to a maximum. This maximum kinetic energy with radiation frequency, but is not affected by intensity.
  3. The number of photoelectrons per second is proportional to the intensity of radiation.
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5
Q

What is the threshold frequency in the photoelectric effect?

A

The frequency of the radiation below which no photoelectrons are emitted.

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6
Q

How does the frequency of the light affect the photoelectric effect?

A

The higher the frequency, the greater the maximum kinetic energy of the photoelectrons.

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7
Q

How does the intensity of the light affect the photoelectric effect?

A

The higher the intensity, the more photoelectrons are emitted per second.

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8
Q

Does the light intensity affect the maximum kinetic energy of photoelectrons?

A

No.

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9
Q

What is light intensity?

A

The power (energy transferred per second) hitting a given area of a metal.

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10
Q

Why can’t the photoelectric effect be explained by wave theory?

A

According to wave theory:
• For a certain frequency, energy is proportional to intensity.
• Therefore each free electron should get a bit of energy from each incoming wave.
• Gradually, each electron would gain enough energy to leave the metal.
So:
• Kinetic energy should increase with intensity - It doesn’t!
• Electrons should be emitted eventually, regardless of frequency - But in reality, there is a threshold frequency!

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11
Q

For a certain frequency of light, the intensity is proportional to…

A

The energy carried.

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12
Q

Why can wave theory not explain why the kinetic energy of a photoelectron in the photoelectric effect only increases with frequency (and not intensity)?

A

As light intensity increases, the energy transferred to each electron should increase also. This doesn’t happen.

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13
Q

Why can wave theory not explain why the threshold frequency in the photoelectric effect?

A

The electrons should gradually gain energy and be emitted eventually, regardless of the radiation frequency. This doesn’t happen.

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14
Q

Who suggested a photon model of light?

A

Einstein

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15
Q

What is a photon?

A

A discrete packet of light.

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16
Q

What is the equation for the energy of a photon?

A

E=hf

Where h = 6.63 x 10^-34

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17
Q

In the photoelectric effect, how many electrons can each photon transfer its energy to?

A

Only one.

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18
Q

How does the photon model of light explain the photoelectric effect?

A
  • When light hits the metal surface, it is bombarded by photons
  • If on of these photons collides with a free electron, the electron will gain energy equal to hf.
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19
Q

What is the work function of a metal?

A

The energy that must be supplied to an electron on the surface of a metal so that it can escape the metal as a photoelectron.

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20
Q

What is the symbol for the work function?

A

Phi ϕ

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21
Q

Is the work function of each metal the same?

A

No, it varies between metals.

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22
Q

What happens when the energy gained by an electron is less than the work function?

A

No electron is emittd, but the metal heats up.

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23
Q

What happens when the energy gained by an electron is more than the work function?

A

The electron is emitted from the metal.

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24
Q

Give an equation for the threshold frequency.

A

f = ϕ / h

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25
Q

Give an explanation for the range in energies of the photoelectrons emitted from a metal.

A

The ones deeper down in the metal lose more energy when exiting than the ones nearer the surface.

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26
Q

What is the photoelectric equation?

A

hf = ϕ + Ek(max)
where:
Ek(max) = 1/2 x m x v(max)^2

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27
Q

What is the equation for the maximum kinetic energy of an electron in terms of mass and velocity?

A

Ek(max) = 1/2 x m x v(max)^2

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28
Q

Why is the kinetic energy of a photoelectron independent of the intensity of light?

A

Each electron can absorb only one photon at a time.

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29
Q

What can the stopping potential be used for?

A

Measuring the maximum kientic energy of photoelectrons.

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30
Q

What is stopping potential?

A

The p.d. needed to stop the fastest moving photoelectrons emitted in the photoelectric effect.

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31
Q

How does calculating the stopping potential work?

A
  • Emitted photoelectrons are made to lose their energy by doing work against a potential difference.
  • The work done by the p.d. in stopping the fastest electrons is equal to the energy they were carrying.
32
Q

What equation is used in calculating the maximum kinetic energy of photoelectrons using their stopping potential?

A

Ek(max) = e x Vs
Where:
e = Charge on the electron (1.6 x 10^-19)
Vs = Stopping potential

33
Q

What is the unit for Ek(max)?

A

Joules (J)

34
Q

Explain why the equation “Ek(max) = e x Vs” works.

A
  • The work done by the potential difference is equal to p.d. x charge.
  • This work done is equal to the kinetic energy lost by the electron.
35
Q

Where in an atom can electrons exist?

A

Only in well-defined energy levels.

36
Q

What n value is the ground state given?

A

n = 1

37
Q

What is the lowest energy state of the atom called?

A

The ground state.

38
Q

What happens when an electron moves down an energy level?

A

A photon is emitted.

39
Q

Why can photons emitted due from an atom only have certain energies?

A

The electrons that cause the emission can only drop between energy levels, so the photons emitted can only take a certain value.

40
Q

Remember to revise energy level diagrams.

A

See revision guide pg 18.

41
Q

Why are electronvolts used instead of Joules when talking about photon energies?

A

The values are so small.

42
Q

What is an electronvolt?

A

The kinetic energy carried by an electron after it has been accelerated through a potential difference of 1 volt.

43
Q

How many Joules is one electronvolt?

A

1.6 x 10^-19

44
Q

How can the energy of an emitted photon be calculated by looking at the drop of the the electron?

A

It is the difference in energies between the two levels.

45
Q

What is excitation?

A

When an electron absorbs a photon of the exact energy and moves to a higher energy level.

46
Q

When can excitation happen?

A

When the photon aborbed by the electron has energy equal to the difference between the two energy levels that the electrons moves between.

47
Q

What is ionisation?

A

When an electron gains enough energy to escape from an atom.

48
Q

What does the energy of each energy level refer to?

A

The amount of energy required to remove an electron at that energy from the atom.

49
Q

What is the ionisation energy of an atom?

A

The amount of energy required to completely remove an electron from an atom from the ground state (n=1).

50
Q

What do fluorescent tubes contain?

A

Mercury vapour at low pressure.

51
Q

How does a fluorescent tube work?

A
  • A high voltage is applied across a tube filled with mercury vapour, which accelerates free electrons
  • Some of the mercury atoms are ionised by collisions, producing more free electrons
  • Other mercury atoms are excited by collisions
  • When these excite electrons return to their ground states, photons in the UV range are emited
  • The phosphorus coating on the inside of the tube absorbs these and re-emits visible photons
52
Q

What coating is placed on the inside of fluorescent tubes?

A

Phosphorus

53
Q

How does the coating on the inside of a fluorescent tube work?

A
  • Atoms in the coating are excited by the UV photons

* When these de-excite, lower energy light photons are emitted.

54
Q

What is a line spectrum?

A

A series of non-continuous colours, where each line corresponds to a particular wavelength of light emitted by a source.

55
Q

What can be used to create a line spectrum?

A
  • Prism

* Diffraction grating

56
Q

A spectrum of white light is…

A

…continuous.

57
Q

What are the two types of light spectra?

A
  • Line spectrum

* Continuous spectrum

58
Q

What is a continous spectrum?

A

A series of unbroken colours, where light of different wavelengths merges into each other without gaps.

59
Q

What can be used to create a continuous spectrum?

A

• Prism

60
Q

What happens when white light is shone through a cool gas?

A
  • At low temperatures, most atoms in the gas are at ground state
  • The electrons can only absorb photons that correspond to differences in energies of energy levels
  • Therefore, photons of specific wavelengths are absorbed and do not show up on the spectrum
61
Q

What is produced when white light is shone through a cool gas?

A

Line absorption spectrum

62
Q

What happens when the absorption and emission spectra of a particular gas are compared?

A

The black lines in the absorption spectrum and bright lines in the emission spectrum match up.

63
Q

What ideas show light as a wave?

A
  • Interference

* Diffraction

64
Q

What ideas show light as a particle?

A

• Photoelectric effect

65
Q

Who came up with the wave-particle duality theory?

A

de Broglie

66
Q

What hypothesis did de Broglie come up with?

A

If wave-like light showed particle properties, particles like electrons should be expected to show wave-like properties.

67
Q

What is the de Broglie equation and what is it used for?

A
  • It is used to calculate the wavelength of a moving particle.
  • It links a wave property (wavelength) with a moving particle property (momentum).
68
Q

What is the equation for the wavelength of a moving particle?

A

λ = h / mv

Where: mv is the momentum (equal to mass x velocity).

69
Q

State an idea that shows the wave nature of electrons.

A

• Electron diffraction

70
Q

How can electron diffraction be observed?

A
  • Electron beam is produced by attracting electrons from a heated filament wire to a +vely charged metal plate with a hole at its centre.
  • Electrons in the vacuum tube interact with the spaces in a graphite crystal.
  • A pattern of rings forms on a fluorescent screen.
71
Q

How does the diffraction pattern of electrons change if the wavelength is increased?

A

The rings are larger and more spaced out.

72
Q

Explain how and why increasing the speed of electrons in electron diffraction changes the diffraction pattern.

A

Increased speed of electrons -> de Broglie wavelength is smaller -> Less diffraction -> Smaller rings

73
Q

In electron diffraction, how can the speed of the electrons be changed?

A

Varying the potential difference between the filament and the metal plate.

74
Q

The wavelength of electrons accelerated in a vacuum tube is about the same as…

A

…electromagentic waves in the X-ray part of the spectrum.

75
Q

How does increasing the mass of particles in diffraction affect the diffraction pattern and why?

A
  • The greater mass reduces the de Broglie wavelength, so less diffraction happens.
  • This gives smaller, tighter rings.
76
Q

Do particles always show wave properties? Why?

A

No, diffraction only happens if the particle interacts with something of a similar size to its de Broglie wavelength. For example, a tennis ball would have a tiny wavelength and could interact with nothing.

77
Q

Explain how the idea of wave-particles duality was developed.

A
  • De Broglie hypothesised wave-particle duality
  • Other scientists had to evaluate the theory (peer review) before he published it
  • Then it was tested with experiments
  • Once there was enough evidence, the theory was validated

A LEVEL - Physics (14 decks)

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