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AQA A Level Physics Paper 1 > Electromagnetic Radiation and Quantum Phenomena > Flashcards

Electromagnetic Radiation and Quantum Phenomena Flashcards

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1
Q

Shining light on a metal can…

A

release electrons

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2
Q

What will happen if you shine light onto the surface of a metal?

A

If you shine light of a high enough frequency onto the surface of a metal, the metal will emit electrons. For most metals this frequency falls in the UV range

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3
Q

What does ‘light’ mean?

A

Light means any EM radiation, not just visible light

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4
Q

Explain the photoelectric effect

A

-Free electrons on the surface of the metal absorb energy from the light
- If an electron absorbs enough energy the bonds holding it to the metal break and the electron is released
- This is called the photoelectric effect and the electrons emitted are called photoelectrons

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5
Q

What are the electrons emitted from the surface of a metal called?

A

Photoelectrons

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6
Q

What is the first main conclusion of the photoelectric effect?

A

For a given metal, no photoelectrons are emitted if the radiation has a frequency below a certain value called the threshold frequency

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7
Q

What is the second main conclusion of the photoelectric effect?

A

The photoelectrons are emitted with a variety of kinetic energies ranging from zero to some maximum value. This value of maximum kinetic energy increases with the frequency of the radiation and is unaffected by the intensity of the radiation

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8
Q

What do the first main two conclusions of the photoelectric effect have in common?

A

They are the two conclusions that had scientists puzzled as they could not be explained using wave theory

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9
Q

What is the third main conclusion of the photoelectric effect?

A

The number of photoelectrons emitted per second is proportional to the intensity of the radiation

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10
Q

What is the intensity of a radiation source?

A

Intensity is the power (the energy transferred per second) hitting a given area of the metal

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11
Q

What is the relationship between the photoelectric effect and wave theory?

A

The photoelectric effect couldn’t be explained by wave theory

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12
Q

What 4 conclusions can be made about the photoelectric effect according to wave theory?

A
  • For a particular frequency of light the energy carried is proportional to the intensity of the beam
  • The energy carried by the light would be spread evenly over the wavefront
  • Each free electron on the surface of the metal would gain a bit of energy from each incoming wave
  • Gradually, each electron would gain enough energy to leave the metal
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13
Q

Following the conclusions made about the photoelectric effect by wave theory, how can it not be explained by wave theory?

A
  • Wave theory suggests that the higher the intensity of the wave the more energy it should transfer to each electron, the kinetic energy should increase with intensity. There is no explanation for the kinetic energy depending only on the frequency
  • There is also no explanation for the threshold frequency. According to wave theory the electrons should be emitted eventually no matter what the frequency is
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14
Q

By which theories could the photoelectric effect be explained by and by which theories could it not be explained by?

A

The photoelectric effect couldn’t be explained by wave theory but it could be explained by Einstein’s photon model of light

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15
Q

Explain Einstein’s photon model of light

A
  • Einstein suggested that EM waves and the energy that they carry exist in discrete packets called photons.
  • The energy carried by one of these photons is : E=hf=hc/λ
  • Einstein saw these photons of light as having a one-to-one particle-like interaction with an electron in a metal surface. A photon would transfer all its energy to one specific electron
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16
Q

What does the photon model of light say about when light hits the surface of a metal?

A
  • When light hits its surface, the metal is bombarded by photons
  • If one of these photons collides with a free electron, the electron will gain energy equal to hf
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17
Q

What does an electron need before it can leave the surface of a metal?

A

Before an electron can leave the surface of a metal it needs enough energy to break the bonds holding it there. This energy is called the work function and its value depends on the metal

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18
Q

What symbol does work function have?

A

Phi (Φ)

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19
Q

Define the electronvolt (eV)

A

The electronvolt is defined as the kinetic energy carried by an electron after it has been accelerated from rest through a potential difference of 1 volt

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20
Q

What is the energy of one electronvolt equal to in joules?

A

1 eV = 1.60*10^-19 J

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21
Q

What is the formula for calculating the energy of one photon and what does each variable in the formula mean?

A

E = hf = hc/λ
- h = Plank’s constant (6.6310^-34 Js)
- c = speed of light in a vacuum (3.0010^8 m/s)

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22
Q

Explain why no photoelectric effect is observed when a surface is given a positive charge?

A

As the surface attracts negative electrons back to its positive surface, therefore the photons now have insufficient energy to release electrons from the surface as the energy required has increased.

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23
Q

What is the proof for showing light as a wave?

A

Interference and Diffraction

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24
Q

How does Interference and Diffraction show light as a wave?

A
  • Light produces interference and diffraction patterns which are alternating bands of dark and light
  • These can only be explained using waves interfering constructively (when two waves overlap in phase) or interfering destructively (when the two waves are out of phase)
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25
Q

What does interference and diffraction show?

A

Light as a wave

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26
Q

What proof shows light behaving as a particle?

A

The photoelectric effect

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27
Q

What does the photoelectric effect show about the behaviour of light?

A

The photoelectric effect shows light behaving as a particle

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28
Q

How does the photoelectric effect show light behaving as a particle?

A
  • Einstein explained the results of photoelectricity experiments by thinking of the beam of light as a series of particle-like photons
  • If a photon of light is a discrete bundle of energy, then it can interact with an electron in a one-to-one way
  • All the energy in the photon is given to one electron
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29
Q

What did De Broglie come up with?

A

The Wave-Particle Duality Theory

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30
Q

What statement did De Broglie make in his PhD thesis?

A

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

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31
Q

What does the De Broglie equation relate?

A

The De Broglie equation relates a wave property (wavelength) to a moving particle property (momentum)

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32
Q

What is each variable in the De Broglie equation?

A
  • λ is wavelength
  • h is Planck’s constant
  • m is mass
  • v is velocity
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33
Q

What can the De Broglie wave of a particle be interpreted as?

A

The De Broglie wave of a particle can be interpreted as a probability wave

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34
Q

What did physicists initially think of De Broglie’s Wave-Particle Duality theory?

A

Many physicists at the time weren’t very impressed - his ideas were just speculation, but later experiments confirmed the wave nature of electrons

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35
Q

What proof shows the wave nature of electrons?

A

Electron Diffraction

36
Q

What does electron diffraction show?

A

The wave nature of electrons

37
Q

How does electron diffraction show the wave nature of electrons?

A

Diffraction patterns are observed when accelerated electrons in a vacuum tube interact with the spaces in a graphite crystal. This confirms that electrons show wave-like properties

38
Q

According to wave theory what can be said about the spread of the lines in the diffraction pattern if the wavelength of a wave is greater?

A

According to wave theory the spread of the lines in the diffraction pattern increases if the wavelength of the wave is greater (assuming the wavelength is still smaller than the gap its diffracting through)

39
Q

In electron diffraction experiments what effect does a smaller accelerating voltage have on the rings in the diffraction pattern?

A

In electron diffraction experiments a smaller accelerating voltage, i.e. slower electrons gives more widely spaced rings

40
Q

What effect does increasing the electron speed have on the diffraction pattern?

A

If you increase the electron speed you increase the electron momentum and the diffraction pattern circles squash together towards the middle. This fits in with the De Broglie equation as if the momentum is greater the wavelength is shorter and the spread of the lines is smaller

41
Q

In general what is the relationship between λ for electrons accelerated in a vacuum tube and the size of electromagnetic waves in the X - ray part of the spectrum?

A

In general, λ for electrons accelerated in a vacuum tube is about the same size as electromagnetic waves in the X - ray part of the spectrum

42
Q

What is the effect of increased particle mass on the diffraction pattern?

A

If particles with a greater mass such as neutrons were travelling at the same speed as electrons they would show a more tightly packed diffraction pattern. This is because a neutron’s mass and therefore its momentum is much greater than an electrons and so a neutron has a shorter de Broglie wavelength

43
Q

When only do you get particle diffraction?

A

You only get diffraction if a particle interacts with an object of about the same size as its De Broglie wavelength

44
Q

Why did De Broglie hypothesise Wave-Particle duality?

A

De Broglie first hypothesised wave-particle duality to explain observations of light acting as both a particle and a wave.

45
Q

Why was De Broglie’s Wave-Particle duality theory not accepted straight away?

A

His wave-particle duality theory wasn’t accepted straight away as other scientists had to evaluate De Broglie’s theory by a process known as peer review before he published it and then it was tested with experiments. Once enough evidence was found to back it up the theory was accepted as validated by the scientific community.

46
Q

How has scientist’s understanding of the nature of matter changed over time and how can this be related to De Broglie’s theory?

A

Scientist’s understanding of the nature of matter has changed over time through the process of hypothesis and validation. De Broglie’s theory is accepted to be true until any new conflicting evidence comes along

47
Q

How does the photon model of light explain the threshold frequency?

A

If the energy gained by an electron on the surface of a metal from a photon is greater than the work function the electron is emitted.
If it isn’t the metal will heat up but no electrons will be emitted. Since for electrons to be released, hf > Φ

48
Q

What is the equation for working out threshold frequency?

A

f = Φ/h

49
Q

How does the photon model of light explain the maximum kinetic energy of emitted electrons?

A

The energy transferred to an electron is hf. The kinetic energy the electron will be carrying when it leaves the metal is hf minus any energy it’s lost on the way out. Electrons deeper down in the metal lose more energy than the electrons on the surface which explains the range of energies.

50
Q

What is the minimum amount of energy an electron can lose?

A

The work function

51
Q

What is the maximum amount of energy a photoelectron can gain and what is the equation used to calculate this energy?

A

The maximum amount of energy gained by a photoelectron is denoted by Ekmax.
- hf = Φ +Ekmax
where Ekmax = 1/2mv^2

52
Q

What is the relationship between the kinetic energy of photoelectrons and the intensity of the light source?

A

The kinetic energy of the electrons is independent of the intensity (the number of photons per second on an area) as they can only absorb one photon at a time. Increasing the intensity just means more photons per second on an area - each photon has the same energy as before

53
Q

The maximum kinetic energy can be measured using the idea of…

A

Stopping potential

54
Q

What is the stopping potential?

A

The emitted electrons from a metal surface are made to lose their energy by doing work against an applied potential difference. This stopping potential, Vs is the pd needed to stop the fast moving electrons with Ekmax.

55
Q

What is the work done by the potential difference in stopping the fastest moving electrons equal to?

A

The energy the electrons were originally carrying

56
Q

What is the formula for calculating stopping potential?

A

eVs = Ekmax
(e is the charge on an electron)

57
Q

How do electrons exist in atoms?

A

Electrons in atoms exist in discrete, well-defined energy levels

58
Q

Which energy level is generally the energy level representing the ground state?

A

The energy level labelled n=1

59
Q

How can electrons move down energy levels?

A

Electrons can move down energy levels by emitting a photon

60
Q

What is the relationship between the transitions between energy levels and the energy an emitted photon can take?

A

Since the transitions of electrons are between definite energy levels the energy of each photon emitted can only take a certain allowed value

61
Q

What is the energy carried by a photon emitted due to an electron moving between energy levels equal to?

A

The energy carried by each photon is equal to the difference in energies between the two levels

62
Q

What is the formula used to calculate the difference in energy between two energy levels and therefore the energy of an emitted photon as a result?

A

(ΔE = E2 - E1) = (hf) = (hc/λ)

63
Q

How can electrons move up energy levels?

A

Electrons can move up energy levels if they absorb a photon with the exact energy difference between the two levels

64
Q

What is the movement of an electron to a higher energy level called?

A

The movement of an electron to a higher energy level is called excitation

65
Q

What can be said about an atom if an electron has been removed from an atom?

A

If an electron has been removed from an atom we say the atom has been ionised

66
Q

What does the energy of each energy level within an atom mean?

A

The energy of each energy level within an atom gives the amount of energy needed to remove an electron in that level from the atom

67
Q

What is the ionisation energy of an atom?

A

The ionisation energy of an atom is the amount of energy needed to completely remove an electron from the atom from the ground state (n=1)

68
Q

What do fluorescent tubes do?

A

Fluorescent tubes use excited electrons to produce light

69
Q

Explain how fluorescent tubes produce light

A

1- Fluorescent tubes contain mercury vapour, across which an initial high voltage is applied. This high voltage accelerates fast-moving free electrons that ionise some of the mercury atoms producing more free electrons
2 - When this flow of free electrons collides with electrons in other mercury atoms, the electrons in the mercury atoms are excited to higher energy levels
3- When these exited electrons return to their ground states, they emit photons in the UV range
4 - A phosphor coating on the inside of the tube absorbs these photons, exciting its electrons to much higher orbits. These electrons then cascade down the energy levels, emitting many lower energy photons in the form of visible light

70
Q

What is the relationship between fluorescent tubes and line emission spectra?

A

Fluorescent tubes produce line emission spectra

71
Q

How do you get a line spectrum from a fluorescent tube?

A

If you split the light from a fluorescent tube with a prism or a diffraction grating you get a line spectrum

72
Q

What is a line spectrum seen as?

A

A line spectrum is seen as a series of bright lines against a black background

73
Q

What does each line on a line emission spectrum correspond to?

A

Each line on a line emission spectra corresponds to a particular wavelength of light emitted by the source. Since only certain photon energies are allowed you only see the wavelengths corresponding to these energies

74
Q

What does shining white light through a cool gas give you?

A

Shining white light through a cool gas gives an absorption spectrum

75
Q

Why in fluorescent tubes is a low pressure mercury vapour used?

A

it is low pressure so the electrons can still accelerate fast enough to have enough energy to collide with mercury atoms

76
Q

Define an excited atom

A

an atom is excited if its electrons occupy higher energy levels with vacancies below

77
Q

What does the emission spectrum show?

A

the frequencies emitted by an excited atom, unique to that element, due to unique energy levels

78
Q

What does the absorption spectrum show?

A

the frequencies emitted that are missing from the continuous spectrum

79
Q

What is a continuous spectrum?

A

A continuous spectrum contains all possible wavelengths

80
Q

What type of spectrum is the spectrum for white light?

A

The spectrum of white light is continuous. If you split the light up with a prism the colours all merge into each other, there aren’t gaps in the spectrum

81
Q

What do hot things emit in the visible and infrared?

A

Hot things emit a continuous spectrum in the visible and infrared

82
Q

Why do hot things emit a continuous spectrum in the visible and infrared?

A

As all the wavelengths are allowed because the electrons are not confined to energy levels in the object producing the continuous spectrum. The electrons are not bound to atoms and are free

83
Q

What is the formula used to calculate the ionisation energy of an atom?

A

Ionisation energy = E1 - E∞

84
Q

What is the relationship between cool gases and the continuous spectrum?

A

Cool gases remove certain wavelengths from the continuous spectrum

85
Q

What type of spectrum do you get when white light passes through a cool gas and why do you get this spectrum?

A

1- You get a line absorption spectrum when light with a continuous spectrum of energy (white light) passes through a cool gas with certain wavelengths removed from the spectrum
2- At low temperatures, most of the electrons in the gas atoms will be in their ground states
3- The electrons can only absorb photons with energies equal to the difference between two energy levels
4- Photons of the corresponding wavelengths are absorbed by the electrons to excite them to higher energy levels
5- These wavelengths are then missing from the continuous spectrum when it comes out on the other side of the gas
6- You see a continuous spectrum with the black lines in it corresponding to the absorbed wavelengths

86
Q

What would happen if you compared the absorption and emission spectra of a particular gas?

A

If you compare the absorption and emission spectra of a particular gas the black lines in the absorption spectrum match up to the bright lines in the emission spectrum

AQA A Level Physics Paper 1 (9 decks)

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