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

Section 2 - EM Radiation and Quantum Phenomena Flashcards

1
Q

What happens if you shine a light of high enough frequency onto the surface of a metal?

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 they will vibrate, 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 4 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 increases with radiation frequency.
  3. Intensity of radiation is the amount of energy per second hitting an area of the metal. The maximum kinetic energy is not affected by intensity.
  4. The number of photoelectrons per second is proportional to the intensity of radiation.
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5
Q

What is the energy of an EM waves of a particular frequency proportional to? (Wave theory)

A

Energy carried should be proportional to the intensity of the beam.

(remember this is only for waves, not for the photoelectric effect)

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6
Q

How is the energy of a wave distributed across it’s wavefront?

A

Spread evenly.

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7
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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8
Q

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

A

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

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9
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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10
Q

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

A

No.

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11
Q

What is light intensity?

A

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

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12
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. If an EM wave had low frequency it would take longer for electrons to gain enough energy but would eventually happen.
So:
• (Kinetic) energy transferred to electron should increase with intensity - It doesn’t!
• Electrons should be emitted eventually, regardless of frequency - But in reality, there is a threshold frequency!
- Kinetic energy depends on only frequency in the photoelectric effect - wave theory can’t explain this.

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13
Q

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

A

The energy carried.

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14
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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15
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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16
Q

Who suggested a photon model of light?

A

Einstein

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17
Q

What is a photon?

A

A discrete packet of light.

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18
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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19
Q

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

A

Only one.

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20
Q

How can the photoelectric effect be demonstrated with a zinc plate, an electroscope and a gold leaf?

A

Zinc plate is attached to the top of an electroscope (box with a piece of metal with a strip of gold leaf attached).

Zinc plate is negatively charged - this 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 (towards the metal).

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21
Q

How does a photocell work?

A
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22
Q

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

A
  • When EM hits the metal surface, it 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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23
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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24
Q

What is the symbol for the work function?

A

Phi ϕ

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25
Q

Is the work function of each metal the same?

A

No, it varies between metals.

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26
Q

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

A

No electron is emitted, but the electron vibrates and releases the energy as another photon.

The metal heats up.

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27
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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28
Q

Give an equation for the threshold frequency.

A

f0 = ϕ / h

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29
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. (For example they might have to do work to get to the surface of the metal)

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30
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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31
Q

What is the reasoning for the equation: hf = ϕ + Ek(max)?

A

The minimun amount of energy an electron can loose is the work function energy.

So the maximum kinetic energy is:

Ek(max) = hf - ϕ
This makes:
hf = ϕ + Ek(max)

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32
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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33
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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34
Q

What is the definition of ionisation energy?

A

minimum energy needed to remove an electron from (an atom from) the ground state.

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35
Q

What can the stopping potential be used for?

A

Measuring the maximum kinetic energy of photoelectrons.

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36
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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37
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.
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38
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

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39
Q

What is the unit for Ek(max)?

A

Joules (J)

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40
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.
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41
Q

Where in an atom can electrons exist?

A

Only in well-defined energy levels.

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42
Q

What n value is the ground state given?

A

n = 1

43
Q

What is the lowest energy state of the atom called?

A

The ground state.

44
Q

What happens when an electron moves down an energy level?

A

A photon is emitted.

45
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.

46
Q

Remember to revise energy level diagrams.

A

See revision guide pg 18 or pg 56 of textbook.

47
Q
A

Step 1) Draw it out

Step 2) Find the energy of the photon, this is also the difference in energy between the two energy levels (ΔE) :

E=hc/λ = hc/656x10^-9 = 3.03x10^-19 Joules

Step 3) Convert into eV:

3.03x10^-19/1.60x10^-19 = 1.895007622 eV

Step 4) IGNORE the negative sign - cross it out

Step 5) We want to the initial energy level (the top one that the electron deexcites from) so use the equation : ΔE = Initial energy
level - Final energy level. Remember we found ΔE and the final E was given in the question:

3.4 = 1.895007622 - initial energy level
initial energy level = 1.895007622 - 3.4
initial energy = -1.50499

Step 6) Convert to Joules again:

-1.50499 x 1.60x10^-19 = -2.4x10^-19 Joules

48
Q

Why will ΔE = Initial energy level - Final energy level?

A

Because it looses energy as it deexcites to final energy level.

49
Q

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

A

The values are so small.

50
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.

51
Q

How many Joules is one electronvolt?

A

1.6 x 10^-19

52
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.

53
Q

What is excitation?

A

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

54
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.

55
Q

What is ionisation?

A

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

56
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.

57
Q

What is the energy carried by a photon (emitted after a transition) equal to?

A

The difference in energies between the two levels of the transition.

58
Q

What equation shows the difference in two energy levels?

A
59
Q
A
60
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).

61
Q

What is the ionisation energy of an unexcited hydrogen atom as seen from the diagram:

A

13.6 eV

62
Q

What is the main component that fluorescent tubes contain?

A

Mercury vapour at low pressure.

63
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 excited electrons return to their ground states, they lose energy as photons in the UV range are emitted.
The photons emitted have a range of energies and wavelengths corresponding to the different transitions of electrons.
• The phosphorus coating on the inside of the tube absorbs these, exciting the electrons to much higher energy levels. Thr electrons cascade down the energy levels and loses energy by emitting lower energy visible photons.

64
Q

What coating is placed on the inside of fluorescent tubes?

A

Phosphorus

65
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.

66
Q

What is a line emission spectrum?

A

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

67
Q

What can be used to create a line spectrum?

A
  • Prism

* Diffraction grating

68
Q

What do line emission spectra provide evidence for?

A
69
Q

What are the two types of light spectra?

A
  • Line spectrum

* Continuous spectrum

70
Q

What is a continuous spectrum?

A

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

71
Q

A spectrum of white light is…

A

…continuous.

72
Q

What can be used to create a continuous spectrum?

A

• Prism

73
Q

Why do hot things emit a continuous spectra?

A

Hot things emit a continuous spectrum in the visible and infrared.

All 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.

74
Q

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

A

Line absorption spectrum

75
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 are missing from the spectrum when it comes out the other side of the gas.
76
Q

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

A

You see a continuous spectra of lines in it corresponding to the absorbed wavelength.

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

77
Q

What ideas show light as a wave?

A
  • Interference

* Diffraction

78
Q

What happens when a beam of light passes through a slit?

A

It spreads out - diffracts

79
Q

Diffraction can only be explained by using…

A

Waves.

80
Q

If light behaved as particles, how would it pass through a slit?

A

light particles would either not get through (if they were too big) or just pass straight through and the beam would be unchanged

81
Q

What ideas show light as a particle-like photon?

A

• Photoelectric effect experiments.

82
Q

How does the photoelectric effect show that light could be a particle?

A

If a photon of light is a discrete bundle of energy, then it can interact with an electron in a one to one way.

83
Q

What did the results from the photoelectric effect prove?

A

That light behaves as a wave and particle = wave-particle duality

84
Q

Who came up with the wave-particle duality theory?

A

de Broglie

85
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.

86
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).
87
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).

88
Q
A

(i) E = hf. This is the energy the electrons will have. This is same as the energy of the photons.

The energy required to remove the electron varies (hence kinetic energy of electrons will vary).

(ii) Work function is the minimum energy needed to release an electron..

Below a certain frequency, energy of the photon is less than the work function (E=hf).

89
Q

How were scientists eventually convinced by de Broglie about wave-particle duality?

A

Peer reviewed before he published it.

Tested through experiments like electron diffraction = provided evidence to be validated by scientists.

90
Q

State an idea that shows the wave nature of electrons.

A

• Electron diffraction

91
Q

How can electron diffraction be observed?

A

Electron diffraction tube:

Electrons are accelerated to high velocities in a vacuum and then passed through graphite crystal.

They diffract through the spaces between the atoms of the crystal.

They produce a pattern of rings.

This provides evidence to show that light has wave properties.

92
Q

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

A

The rings are larger and more spaced out.

93
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

94
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.

95
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.

96
Q

When do you only get diffraction?

A

If a particle interacts with an object of about the same size as it’s de Broglie wavelength.

97
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.

This is because higher mass means higher momentum = shorter de Broglie wavelength.

98
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.

99
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
100
Q
A

hf is the same energy from photons.

energy required to remove the electron varies (hence kinetic energy of electrons will vary).

101
Q

What does diffraction effect on an image?

A

blur

102
Q

If you want less blur on an image what should you consider?

A

Shorter wavelength to resolve tiny detail

103
Q

How can you have less diffraction in microscopes?

A

Use electron waves - light blurs out details more than electron waves.
Electron microscope can resolve finer details -e.g. they can see strands of DNA.

A Level Physics (13 decks)

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