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Physics > 2.2 EM radiation and quantum phenomena > Flashcards

2.2 EM radiation and quantum phenomena Flashcards

1
Q

are EM waves transverse or longitudinal

A

transverse

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2
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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3
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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4
Q

What is the threshold frequency

A

The minimum frequency of light required for an electron to be emitted

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5
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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5
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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6
Q

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

A

No.

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7
Q

What is light intensity?

A

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

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8
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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9
Q

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

A

The energy carried.

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10
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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11
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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12
Q

Who suggested a photon model of light?

A

Einstein

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13
Q

What is a photon?

A

A discrete packet of light.

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14
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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15
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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16
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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17
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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18
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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19
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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20
Q

What n value is the ground state given?

A

n = 1

21
Q

What happens when an electron moves down an energy level?

A

A photon is emitted.

22
Q

How does a fluorescent tube work?

A
  • High voltage applied across mercury vapour accelerates fast moving free electrons which collide with the mercury atoms
  • Mercury electrons are excited and then return to the ground state, releasing a UV photon
  • The tube’s phosphorus coating absorbs the UV photons and it’s electrons are excited, they cascade down the energy levels and emit visible photons
23
Q

What coating is placed on the inside of fluorescent tubes?

A

Phosphorus

24
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.

25
Q

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

A

Line absorption spectrum

26
Q

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

A

It spreads out - diffracts

27
Q

What did the results from the photoelectric effect prove?

A

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

28
Q

Who came up with the wave-particle duality theory?

A

de Broglie

29
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.

30
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

31
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.

32
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.

33
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.

34
Q

xplain how the idea of wave-particles duality was develope

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
35
Q

What does diffraction effect on an image?

A

blur

36
Q

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

A

Shorter wavelength to resolve tiny detail

37
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.

38
Q

What can be used as evidence for the discrete energy levels in atoms?

A

Line emission and absorption spectra as the lines appear at discrete points which show where a light photon of specific wavelength and frequency has been absorbed or emitted, this shows electrons can only absorb an exact amount of energy to be excited to the next discrete energy level

39
Q

What is a wave particle duality?

A

All particles have both particle and wave properties, waves can have particle properties e.g. light acts as a particle in the photoelectric effect and as a wave when it is diffracted

40
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.

41
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.
42
Q

State an idea that shows the wave nature of electrons.

A

Electron diffraction

43
Q

How does a photocell work?

A
44
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.

45
Q

What can the stopping potential be used for?

A

Measuring the maximum kinetic energy of photoelectrons

46
Q

What is stopping potential?

A

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

47
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.

48
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.

49
Q

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

A

The rings are larger and more spaced out.

Physics (43 decks)

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