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Physics > Section 2: Electromagnetic Radiation and Quantum Phenomena > Flashcards

Section 2: Electromagnetic Radiation and Quantum Phenomena Flashcards

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1
Q

If you shine radiation of a high enough frequency onto the surface of a metal, what happens?

A

It will instantly emit electrons.
For most metals, the necessary frequency falls in the ultraviolet range.

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2
Q

If you shine radiation of a high enough frequency onto the surface of a metal, it will instantly emit electrons. Why does this happen?

A

Because of the way atoms are bonded together in metals, metals contain ‘free electrons’ that are able to move about the metal. The free electrons on or near the surface of the metal absorb the energy from the radiation, making them vibrate. If an electron absorbs enough energy, the bonds holding it to the metal can break and the electron can be released.

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3
Q

During the photoelectric effect, what are the electrons that are emitted called?

A

photoelectrons

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4
Q

How does the photoelectric effect work?

A

If you shine radiation of a high enough frequency onto the surface of a metal, it will instantly emit electrons. Because of the way atoms are bonded together in metals, metals contain ‘free electrons’ that are able to move about the metal. The free electrons on or near the surface of the metal absorb the energy from the radiation, making them vibrate. If an electron absorbs enough energy, the bonds holding it to the metal can break and the electron can be released.

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5
Q

What are the 4 main conclusions from the photoelectric effect?

A
  1. For a given metal, no photoelectrons are emitted if the radiation has a frequency below a certain value - called the threshold frequency.
  2. 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.
  3. The intensity of radiation is the amount of energy per second hitting an area of the metal. The maximum kinetic energy of the photoelectrons is unaffected by varying the intensity of the radiation.
  4. The number of photoelectrons emitted per second is proportional to the intensity of the radiation.
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6
Q

What is conclusion 1 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 conclusion 2 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.

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8
Q

What is conclusion 3 of the photoelectric effect?

A

The intensity of radiation is the amount of energy per second hitting an area of the metal. The maximum kinetic energy of the photoelectrons is unaffected by varying the intensity of the radiation.

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9
Q

What is conclusion 4 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 did the photoelectric effect prove?

A

Light can’t just be a wave as certain observations of the photoelectric effect can’t be explained by the classical wave theory.

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11
Q

What does the wave theory state about the frequency of electromagnetic waves and their energy?

A

For a particular frequency of electromagnetic waves, the energy carried should be proportional to the intensity of the beam. The energy carried by the electromagnetic wave would also be spread evenly over the wavefront.

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12
Q

If an electromagnetic wave were shone on a metal, what does the wave theory state should happen? Why is this not accurate?

A

If an electromagnetic wave were shone on a metal, each free electron on the surface of the metal would gain a bit of energy from each incoming wavefront. Gradually, each electron would gain enough energy to leave the metal. If the electromagnetic wave has a lower frequency, it would take longer for the electrons to gain enough energy, but it would eventually happen.
However, electrons are never emitted unless the wave is above a threshold frequency - so wave theory can’t explain the threshold frequency.

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13
Q

What does the kinetic energy of the photoelectrons depend on?

A

Wave theory can’t explain the fact that the kinetic energy depends only on the frequency in the photoelectric effect.

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14
Q

Who suggested that electromagnetic waves can only be released in discrete packets, or quanta?

A

Max Planck

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15
Q

How do you find the energy of one wave-packet?

A

E = hf
E = hc/wavelength

E is energy in J
h is Planck’s constant (6.63x10-34) in Js
f is frequency in Hz
c is speed of light in a vacuum (3x10 8) in m/s

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16
Q

What is Planck’s constant?

A

6.63x10-34 Js

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17
Q

What did Einstein suggest following Planck’s theory?

A

Einstein suggested that electromagnetic waves (and the energy they carry) can only exist in discrete packets. He called these wave-packets photons.

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18
Q

What did Einstein suggest about photons?

A

He saw these photons of light as having a one-on-one, particle-like interaction with an electron in a metal surface. Each photon would transfer all its energy to one specific electron.

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19
Q

What was so significant about Einstein’s photon model?

A

The photon model could be used to explain the photoelectric effect.

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20
Q

How can the photoelectric effect be demonstrated with an experiment?

A

A zinc plate is attached to the top of an electroscope (a box containing a piece of metal with a strip of gold leaf attached).
The zinc plate is negatively charged (which in turn means the metal in the box is negatively charged). The negatively charged metal repels the gold leaf, causing it to rise up.
UV light is then shone onto the zinc plate. The energy of the light causes electrons to be lost from the zinc plate via the photoelectric effect.
As the zinc plate and metal lose their negative charge, the gold leaf is no longer repelled and so falls back down.

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21
Q

What is meant by the work function energy?

A

When EM radiation hits a metal, the metal’s surface is bombarded by photons. If one of these photons collides with a free electron, the electron will gain energy equal to hf.
Before an electron can leave the surface of the metal, it needs enough energy to break the bonds holding it in there. This energy is called the work function energy.

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22
Q

What does the work function energy depend on?

A

the metal the electrons are part of

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23
Q

If the energy gained by the electron from a photon is greater than the work function energy, what happens?

A

the electron can be emitted

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24
Q

If the energy gained by the electron from a photon is lower than the work function energy, what happens?

A

The electron will just shake about a bit, then release the energy as another photon. The metal will heat up, but no electrons will be emitted.

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25
Q

How do you find threshold frequency?

A

threshold frequency = work function/Planck’s constant

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26
Q

Why do electrons that have been emitted from a metal have a range of kinetic energies?

A

The energy transferred for EM radiation to an electron is the energy it absorbs from one photon (hf). The kinetic energy it will be carrying when it leaves the metals is hf - any other energy losses. These energy losses are the reason the electrons emitted from a metal have a range of kinetic energies.

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27
Q

What is the photoelectric equation?

A

hf = work function + maximum kinetic energy

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28
Q

How do you find the maximum kinetic energy a photoelectron can have?

A

maximum kinetic energy = 1/2 x mass of an electron x maximum velocity squared

29
Q

What is the mass of an electron?

A

9.11 x 10-31 kg

30
Q

What does it mean by the intensity?

A

the number of photons per second on an area

31
Q

Is there a link between kinetic energy and the intensity?

A

The kinetic energy of the electrons is independent of the intensity, 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.

32
Q

What is stopping potential?

A

The maximum kinetic energy can be measured using the idea of stopping potential. Photoelectrons emitted by the photoelectric effect can be made to lose their energy by doing work against an applied potential difference. The stopping potential is the potential difference needed to stop the fastest moving electrons travelling with kinetic energy. The work done by the potential difference in stopping the fastest electrons is equal to the energy they were carrying.

33
Q

How do you find the stopping potential?

A

e (charge of an electron) x V = KE

34
Q

What is the electron volt defined as?

A

the kinetic energy carried by an electron after it has been accelerated from rest through a potential different of 1 volt.

35
Q

How many joules are in 1 electron volt?

A

1.6 x 10-19

36
Q

What is the ground state in energy levels?

A

The lowest energy level an electron can be in

37
Q

When electrons move down an energy level what do they emit?

A

They emit an photon

38
Q

When transitioning between definite energy levels, what does this mean about the energy of the emitted photons?

A

the energy of each photon emitted can only take a certain allowed value.

39
Q

The energy carried by a photon emitted after a transition is equal to what?

A

Equal to the difference in energies between the two levels of the transition.

40
Q

When can electrons move up energy levels when they absorb a photon?

A

The electron needs to absorb the exact energy difference between the two levels.

41
Q

What is excitation?

A

the movement of an electron to a higher energy level

42
Q

What is ionisation?

A

When an electron has been removed from at atom, the atom is ionised.

43
Q

What is the ionisation energy of an atom?

A

The amount of energy needed to remove an electron from the ground state atom.

44
Q

How do fluorescent tubes use photon emission to work?

A

Fluorescent tubes use the excitation of electrons and photon emission to produce visible light. They contain mercury vapour, across which a high voltage is applied. This high voltage accelerates fast-moving free electrons that ionise some of the mercury atoms, producing more free electrons. When this flow of free electrons collides with the electrons in the mercury atoms, the atomic mercury electrons are excited to a higher energy level. When these excited electrons return to their ground states, they lose energy by emitting high-energy photons in the UV range. The photons emitted have a range of energies and wavelengths that correspond to the different transitions of the electrons. A phosphor coating on the inside of the tube absorbs these photons, exciting its electrons to much higher energy levels. These electrons then cascade down the energy levels and lose energy by emitting many lower energy photons of the visible light.

45
Q

How do you produce a line spectrum?

A

if you split the light from a fluorescent tube with a prism or a diffraction grating, you get a line spectrum. Diffraction gratings and prisms work by diffracting light of different wavelengths at different angles.

46
Q

Do diffraction gratings or prisms produce more clearer spectral lines?

A

A diffraction grating produces much clearer and more defined spectral lines than a prism.

47
Q

What is a line emission spectrum?

A

It can be seen as a series of bright lines against a black background. Each line corresponds to a particular wavelength of light emitted by the source.

48
Q

What do line spectra provide evidence for?

A

They provide evidence that the electrons in atoms exist in discrete energy levels. Atoms can only emit photons with energies equal to the difference between two energy levels. Since only certain photon energies are allowed, you only see the corresponding wavelengths in the line spectrum.

49
Q

What does the continuous line absorption spectra look like?

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 any gaps in the spectrum.

50
Q

What kind of spectrum do hot things emit?

A

They emit a continuous spectrum in the visible and infrared. 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.

51
Q

What are line absorption spectra?

A

You get a line absorption spectrum when light with a continuous spectrum of energy (white light) passes through a cool gas. At low temperatures, most of the electrons in the gas atoms will be in their ground states. Photons of the correct wavelength are absorbed by the electrons to excite them to higher energy levels. These wavelengths are the missing from the continuous spectrum when it comes out the other side of the gas. You see a continuous spectrum with a black lines in it corresponding to the absorbed wavelength.

52
Q

How do you compare line absorption spectra and emission spectra?

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.

53
Q

What is diffraction?

A

When a beam of light passes through a narrow gap, it spreads out.

54
Q

What is the only explanation of diffraction?

A

Waves

55
Q

If light was acting as a particles, what would happen is it was passed through a narrow gap?

A

If the light was acting as a particles, the light particles in the beam would either not get through the gap (if they were too big), or just pass straight through and the beam would be unchanged.

56
Q

What are examples of the phenomenon known as wave-particle duality?

A

The photoelectric effect and diffraction showing that light behaves as both a particle and a wave.

57
Q

How do you find the De Broglie wavelength?

A

Wavelength = Planck’s constant / momentum (mv)

58
Q

What did De Broglie’s theory suggest?

A

He said if ‘wave-like’ light showed particle properties (photons), ‘particles’ like electrons should be expected to show a wave-like properties.

59
Q

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

A

a ‘probability wave’

60
Q

What has to happen with De Broglie’s theory before he was allowed to publish it?

A

His theory wasn’t accepted straight away. Other scientists had to evaluate De Broglie’s theory by a process known as peer review before he published it. It was then tested with experiments such as electron diffraction. Once enough evidence was found to back it up, the theory was accepted as validated by the scientific community.

61
Q

How does electron diffraction provide evidence that electrons have wave properties and therefore supports de Broglie’s theory?

A

Diffraction patterns can be observed using an electron diffraction tube. Electrons are accelerated to high velocities in a vacuum and then passes through a graphite crystal. As they pass through the spaces between the atoms of the crystal, they diffract just like waves passing through a narrow slit and product a pattern of rings.

62
Q

How can you alter the electron diffraction experiment to change the size of the rings produced?

A

A smaller accelerating voltage, i.e. slower electrons, gives widely spaced rings.
Increase the electron speed and the diffraction pattern circles squash together towards the middle.
If the velocity is high, the wavelength is shorter and the spread of lines is smaller.

63
Q

How do you produce widely spaced rings in the electron diffraction experiment?

A

a smaller accelerating voltage (so slower electrons) gives widely spaced rings

64
Q

How do you produce narrowly spaced rings in the electron diffraction experiment?

A

Increase the electron speed (provide a larger voltage to the system)

65
Q

As you increase velocity of the electron in the electron diffraction experiment, what does this affect wavelength and the spread of the lines?

A

velocity is higher
wavelength is shorter
spread of lines is smaller

66
Q

You only get a diffraction if a particle interacts with an object of about the size of what?

A

the same size as its De Broglie wavelength

67
Q

How does the mass of the particles travelling in the electron diffraction experiment affect the diffraction pattern?

A

If particles with a greater mass (e.g. neutrons) were travelling at the same speed as the electrons, they would show a more tightly-packed diffraction patterns. This is because a neutron’s mass (and therefore momentum) is much greater than an electron’s, and so a neutron has a shorter De Broglie wavelength.

68
Q

Why do electron microscopes allow you see things in much more detail than light microscopes?

A

A shorter wavelength gives smaller diffraction effects.
Diffraction effects blur detail on an image. If you want to resolve tiny detail in an image, you need a shorter wavelength.
Light blurs out detail more than ‘electron-waves’ do, so an electron microscope can resolve finer detail than a light microscope.

Physics (15 decks)

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