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A-Level Physics > 2. Electromagnetic Radiation and Quantum Phenomena > Flashcards

2. Electromagnetic Radiation and Quantum Phenomena Flashcards

1
Q

What device was used to achieve the ‘photoelectric’ effect?

A

gold leaf electroscope

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2
Q

What does a gold leaf electroscope consist of?

A

a metal plate attached to a rigid metal pole with a flexible piece of gold foil attached

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3
Q

What is the charge of a gold leaf electroscope?

A

negatively charged

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4
Q

How is a gold leaf electroscope made to be negatively charged?

A

adding extra electrons

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5
Q

Why is the flexible gold foil repelled from the metal pole?

A

both are negatively charged

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6
Q

When does the gold foil fall back down?

A

when light of a certain frequency is shone onto the metal plate

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7
Q

Why does the gold foil fall back down when light of a certain frequency is shone onto the metal plate?

A

because the light was causing electrons to be released from the metal causing the apparatus to become less negatively charged meaning the force of repulsion weakened

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8
Q

Why doesn’t the experiment work if charged positively?

A
  • positive charges are fixed in the metal and can’t be released like the electrons
  • if the metal is positively charged, then the negative electrons are attracted back to the plate and so it is harder for them to escape
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9
Q

What three things did wave theory predict?

A
  1. any frequency (colour) of light should cause the photoelectric effect
  2. increasing the intensity (brightness) of the light should increase the energy of the electrons emitted
  3. it should take longer for electrons to be emitted when using low intensity light compared to high intensity light
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10
Q

Regarding theory 1, what actually happened?

A

only above a certain ‘threshold’ frequency were electrons emitted

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11
Q

What was the conclusion made about the 1st theory?

A
  • energy must be delivered in packets (particles), and must be proportional to the frequency of the wave
  • electrons can’t store energy
  • must be delivered all in one go
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12
Q

Regarding theory 2, what actually happened?

A

as long as you are above a certain threshold frequency, increasing intensity increases the amount of electrons but their individual energy stayed the same

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13
Q

What was the conclusion made the 2nd theory?

A
  • there is a one-to-one interaction between a photon and an electron
  • the greater the intensity the more photons produced, therefore the greater the number of electrons emitted
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14
Q

Regarding theory 3, what actually happened?

A

the electrons were emitted instantly regardless of the intensity of the light

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15
Q

What was the conclusion made about the 3rd theory?

A
  • energy must be delivered in packets in one go, rather than continuously
  • electrons can’t store energy
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16
Q

What is required for an electron to be released from the surface of a metal?

A

energy

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17
Q

What is the work function?

A

the minimum amount of energy needed for an electron to escape the surface of a metal

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18
Q

Is the work function the same or different for different metals?

A

different

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19
Q

What supplies the energy needed for electrons to escape?

A

photons

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20
Q

What happens when photons have less energy than the work function?

A
  • nothing will happen
  • increasing the intensity of the light (producing more photons) will also have no effect
  • each electron can only interact with one photon
  • they can’t store energy
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21
Q

What happens when photons have the same energy as the work function?

A
  • electrons is released
  • but with no kinetic energy
  • the frequency of this photon is known as the ‘threshold frequency’
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22
Q

What is the threshold frequency?

A

the minimum frequency needed for an electron to escape the surface of a metal

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23
Q

What is the equation for threshold frequency?

A

threshold frequency = work function/Planck’s constant

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24
Q

What happens when photons have more than the energy of the work function?

A
  • electron is released
  • it leaves faster as the extra energy is transformed into kinetic energy
  • we don’t release extra electrons as it is still only a 1 to 1 interaction
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25
Q

What is the equation for the photoelectric effect?

A

hf = work function + max kinetic energy

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26
Q

Why are electrons emitted with a range of speeds?

A

those at deeper levels require more energy to escape

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27
Q

What is intensity?

A

the amount of energy arriving every second per unit area

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28
Q

How can you cause a gas to glow?

A

heat it up/excite it

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29
Q

How can you heat up a gas?

A

by passing a very high current through it

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30
Q

What is this high current made up of?

A

fast-moving electrons

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31
Q

What is the single colour we see made up of?

A

multiple photons

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32
Q

How can we see the multiple photons?

A

by splitting the light either by using a prism or a diffraction grating

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33
Q

What is an emission spectrum?

A

the photons that get emitted

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34
Q

What does a continuous spectrum consist of?

A

all the visible wavelengths

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35
Q

What happens when you pass white light through the same gas, but cold (de-excited)?

A

it blocks certain photons

36
Q

What are the photons that the gas blocks?

A

photons of the same wavelength as the ones it emitted

37
Q

What causes elements to both emit AND absorb very specific photons?

A

electrons

38
Q

What can the electrons around an atom only exist at?

A

certain levels of energy

39
Q

How do electrons move up and down energy levels?

A
  • they can absorb energy and move up

- they can emit energy and move down

40
Q

Can electrons exist between energy levels?

A

no - it is the forbidden zone

41
Q

What is level 1 known as?

A

the ground state (n=1)

42
Q

Where do electrons need to receive the most energy to escape the atom?

A

level 1/ground state

43
Q

Where are the electrons most stable?

A

ground state/level 1

44
Q

What is (n=∞)

A

ionisation level

45
Q

What does the ionisation level represent?

A

when an electron has left the atom entirely, turning the atom into an ion

46
Q

How can electrons move up and down energy levels?

A

by gaining and losing energy

47
Q

How can an electron gain energy?

A
  • absorb a photon (1 to 1 interaction)

- get hit with an external electron

48
Q

What is ionisation?

A

if an electron gets enough energy it can leave the atom entirely

49
Q

When is an electron excited?

A
  • when it is in an energy level higher than the ground state
  • the atom is now an ‘excited’ atom
50
Q

What is the ionisation energy?

A

the amount of energy needed to leave the atom from the ground state

51
Q

When can an electron only absorb a photon?

A

when the photon gives the electron exactly the right amount of energy to move up one or more whole levels

52
Q

Technically, how much energy does an electron have when it has left the atom?

A

zero energy (used it all up to escape)

53
Q

Do we mark down the energy of each level as positive or negative?

A

negative

54
Q

How much energy do we mark down (n=∞) as having?

A

zero

55
Q

Is energy gained or lost moving down energy levels?

A

moving down energy levels loses you energy so you get further from 0 (more negative)

56
Q

Which element is most used for energy level diagrams in exams?

A

hydrogen

57
Q

Why do elections not like being outside of the ground state?

A

the ground state is where they have the least amount of energy and are the most stable?

58
Q

How do the electrons return to the ground state?

A

by emitting energy

59
Q

How is this energy emitted?

A

as photons

60
Q

How can electrons return to the ground state?

A

either in a single ‘jump’ or they will cascade

61
Q

What happens when excited electrons ‘de-excite’?

A
  • they will release photons with energy equal to the energy different between levels
  • meaning that the exact same photons are emitted that can be absorbed
62
Q

What do the absorption and emission spectrum equal when combined?

A

a continuous spectrum

63
Q

What do fluorescent tubes use energy levels to produce?

A

white light

64
Q

How do fluorescent tubes work?

A
  1. fluorescent tubes are filled with mercury vapour
  2. a high p.d. is applied across the tube
  3. this causes free electrons to rapidly accelerate from one side to the other
  4. the free electrons collide with the ground state electrons in the mercury, transferring energy
  5. this excites them to a higher energy level
  6. the excited electron will return to the ground state, releasing a UV photon, which is not visible to the naked eye
  7. the reason a high energy UV photon is released is because the energy gaps in mercury are large
  8. the tube is coated with a fluorescent coating
  9. this turns the UV photons into visible light
  10. the UV photons will excite the electrons in the ground state of the coating to a higher energy level
  11. the electron cascades back down to the ground state, releasing lower energy photons which are in the visible part of the spectrum, appeasing white to our eyes
  12. the energy gaps are smaller so the photons emitted have a lower energy
65
Q

What is the photoelectric effect evidence of?

A

light behaving as a particle

66
Q

What are the particles of light called?

A

photons (packets of waves)

67
Q

What is diffraction an example of?

A

light acting like a wave

68
Q

What happens when light passes by an object?

A

the light ‘bends’ around the object

69
Q

What is wave particle duality?

A

the fact that light shows particles like behaviour in certain scenarios and wave like behaviour in others

70
Q

What did scientist Louis De Broglie suggest?

A

that if waves can behave like particles, then surely particles can behave like waves

71
Q

What is the equation De Broglie developed to allow you to work out the wavelength of a particle?

A

λ = h/mv = h/p

72
Q

How can you prove that De Broglie is correct?

A

by observing a particle doing something that only a wave can do - diffraction

73
Q

When do waves show the greatest amount of diffraction?

A

when the wavelength is similar in size to the gap/object they diffract around

74
Q

When can an electron be shown to diffract?

A

when going at a great enough speed

75
Q

How is an electron made to travel at great enough speeds to diffract?

A
  1. accelerating electrons using an electron gun
  2. then passing them through a thin graphite screen
  3. graphite has very regular spacings between atoms - these are similar in size to the De Broglie wavelength of the electrons
  4. the electrons impact a fluorescent screen; where they shit, the screen will glow
76
Q

What would you expect to see if electrons behaved only like particles?

A

a single point of green light on the screen

77
Q

What do you see instead?

A

a series of rings

78
Q

What are these rings caused by?

A

electrons diffracting through the graphite

79
Q

What are the bright spots and dark rings caused by?

A

the electrons constructively and destructively interfering

80
Q

What happens when the electrons are slow?

A
  • slow electrons = bigger wavelength
  • bigger wavelength = more diffraction
  • more diffraction = more spaced out rings
81
Q

What happens when the electrons are fast?

A
  • fast electrons = smaller diffraction
  • smaller wavelength = less diffraction
  • less diffraction = less spaced out rings
82
Q

When does light display particle-like behaviour? What is this evidence of?

A
  • photoelectric effect

- existence of threshold frequency

83
Q

When does light display wave-like behaviour? What is this evidence of?

A
  • diffraction

- interference pattern seen

84
Q

When do electrons display particle-like behaviour? What is this evidence of?

A
  • deflection in a magnetic or electric field

- curved track of electron

85
Q

When do electrons display wave-like behaviour? What is this evidence of?

A
  • electron diffraction

- interference pattern seen

A-Level Physics (15 decks)

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