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Aqa Physics > Em Radiation And Quantam Phenomena > Flashcards

Em Radiation And Quantam Phenomena Flashcards

(55 cards)

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1
Q

What is the photoelectric effect ?

A

The emission of photoelectrons from the surface of a metal when light with a frequency greater than the threshold frequency is shone on it .

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2
Q

What is the threshold frequency for the photo electric effect ?

A

The minimum frequency required to liberate an electron from the surface of the metal

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3
Q

How are electrons emitted in the photoelectric effect ?

A
  • The electrons absorb the energy from the uv radiation - results in them vibrating .
  • if sufficient energy is provided the bond holding the electron to the atom can be broken releasing an electron
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4
Q

What is an example of the photoelectric effect ?

A

The gold leaf electroscope

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5
Q

How did the photoelectric effect disprove that light was a wave ?

A

If a wave of energy had been shone - the energy must have been distributed amongst all the electrons
- this meant that there must have been a time delay as the electrons would have had to absorb sufficient energy
-however the process was instantaneous regardless of the frequency / brightness

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6
Q

What happens if you increase the frequency of the light shine on the metal surface ?

A

They gain more energy - as e is directly proportional to the frequency ( more kinetic energy )

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7
Q

What happens if you increase the intensity of the light shone one the metal surface ?

A

The amount of photoelectrons emitted increases due to the interaction being a one to one interaction ( one photon = one electron emitted )

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8
Q

What is the work function of a metal ?

A

It is the minimum energy required to release a photoelectric from the surface of a metal

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9
Q

What happens if the energy gained by the photon is lower than the work function ?

A

The electron would vibrate and then release the energy as another photon - metal will heat up

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10
Q

What must the energy gained by the electron be in order to be released?

A

> work function or equal to

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11
Q

How does a vacuum photocell work ?

A

When the emitter plate ( photocathode ) is exposed to em radiation with a frequency greater than the threshold frequency photo electrons are ejected.

The ( anode ) collector plate collects the photoelectrons and the sensitive ammeter indicates the presence of a tiny current

Current is proportional to the number of photoelectrons emitted per second

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12
Q

What is an electron volt ?

A

The energy gained by an electron moving across a potential difference of 1 volt

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13
Q

How to convert from ev to joules ?

A

Multiply by 1.6*10^-19

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14
Q

How to convert from joules to electron volt ?

A

Divide by 1.6*10^-19

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15
Q

How can electrons within an atom exist ?

A

in discrete energy levels

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16
Q

What is the lowest energy level an electron can be in ?

A

N=1 , ground state

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17
Q

When is an electron in the excited stage ?

A

When the electron is on energy level above the ground state but below the ionisation level ( n= ♾️ )

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18
Q

What can occur if an electron absorbs a photon of energy E ?

A
  • sufficient energy = electron leaves the atom (ionised ) leaving a positive ion behind

-absorbs the energy and gets excited - move to a higher energy level

  • not sufficient energy = electron vibrates - then reemits the energy as another photon
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19
Q

What is ion ?

A

When a charged particle , created by the addition or removal of electrons

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20
Q

What is ionisation?

A

The removal or addition of an electron from an atom when given sufficient energy = ionisation

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21
Q

What is ionisation energy ?

A

The amount of energy required to remove an electron from the atom

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22
Q

What is excitation ?

A

When an electron is provided with enough energy to move to a higher energy level , but not enough to leave the atom

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23
Q

What is the excitation energy ?

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A

The energy required to move an electron from a lower state to a higher state

24
Q

What happens if the energy of the photon is not equal to the amount of required by an electron to move to another energy level what happens ?

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A

The photon isn’t absorbed by the electron , but passes straight through it

25
When will excitation occur ?
When the energy of the photon is exactly equal to the difference in energy of the initial and final energy level.
26
Why does an electron de excite ?
An electron in the excited stater is typically unstable
27
What happens when an electron de-excites ?
An electron moves to a lower energy level, emitting energy as a photon of electromagnetic radiation
28
What would the energy of a photon during excitation be equal to ?
The difference in the initial and final energy level - meaning only specific wavelengths can be emitted
29
What is meant by ground state ?
When electrons are in their lowest energy level , most stable state
30
Explain why only photons of certain frequencies can result in excitation within a particular atom ?
- electron exist in discrete energy levels - need to absorb exact amount of energy to move to a higher energy level - photons with certain frequency are able to provide this energy to the electron in a one to one interaction ( E = hf ) - the energy from the photon is absorbed by the electron
31
What is fluorescence ?
It is the emission of visible light by a substance that has absorbed light or other electromagnetic radiation.
32
What is the construction of a fluorescent tube ?
- glass tube - partially evacuated and filled with low pressure mercury vapour - inner phosphorus coating ( fluorescent chemical )
33
How does a fluorescent tube work
- When a current is applied there would be a flow of free electrons - the free electrons collide with the mercury atoms exciting them when they transfer energy ( via collisions ) - as the mercury atoms are unstable when they are excited , they de excite emitting uv photons - the uv photons excite the phosphorus electrons - as they de excite they emit photons ( low energy visible light photons , producing a fluorescent glow
34
Explain how the excited mercury atoms emit photons
The excited mercury atoms de excite back to a lower energy level as they are unstable. In doing so they emit photons of energy equal to the difference in energies between the levels
35
Explain how the mercury atoms become excited in a fluorescent tube.
Free electrons collide with the mercury atoms , transferring energy to the mercury electrons. This causes them to excite and move to a higher energy level.
36
Explain how the ultraviolet photons in the tube are converted into photons in the visible light part of the em spectrum ?
- The uv photons are absorbed by the phosphorus - excitation - the phosphorus electrons de excite indirectly to the previous lower level - emitting low energy visible photons
37
How would you calculate the frequency of the photons emitted when an electron falls to the ground state from an given energy level ?
(E1 - E2 ) / h ( planks constant ) Making sure to convert to joules
38
When is a continuous spectrum produced ?
When excited electrons within hot dense objects such as the interior of stars or the filament lamp de excite , they emit light of all colours ( wavelengths ) producing a continuous spectrum - no spectral lines
39
When is a line absorption spectra produced ?
When light from a hot dense object is passed through a cool cloud of gas, photons of certain wavelengths are absorbed by the electrons in the cool gas cloud , producing an absorption spectrum.
40
Which photons will be absorbed when light from a hot dense object is passed through a cool cloud of gas ?
- photons with energies equal to the difference of two energy levels - spectral lines corresponding to the absorbed wavelength
41
When is an emission spectrum produced ?
When the excited electrons from the hot gas de-excite and emit photons of particular wavelengths, an emission spectrum is produced
42
How can the line emission and absorption spectrums be used to identify elements ?
The absorption lines and emissions lines would be specific to elements
43
How do collisions of charged particles with gas atoms cause the atoms to emit photons ?
- energy from the charged particles transfer to electrons in gas molecules - electron are excited - higher energy state - the more energy the greater energy level reached - electrons unstable - de excites emitting a photon
44
How do spectral lines explain the concept of discrete energy levels ?
The photon energy is proportional to the frequency - Spectral lines at specific wavelengths - each spectral line corresponds to an electron falling down to lower energy state - energy gap = hc/wavelength - larger energy gap ( higher energy photon is emitted so shorter wavelength)
45
What evidence tells us that light is a wave ?
- when light passes through a small gap , it diffracts. Diffraction is evidence that light is a wave
46
What evidence tells us that light is a particle?
When light of a high enough frequency is shone on the metal , photoelectric emission is seen. This is evidence that light is a particle
47
What is the wave- particle duality ?
It is when light can behave as a particle and a wave
48
What did Louis de brogie say ?
If waves have particulate properties, it was reasonable to suppose that particles had wave properties
49
How do you get the debrogie wavelength equation ?
By equating E = hf to E = mc^2
50
Why can we not diffract ?
As the wavelength is inversely proportional to the mass - as we have a large mass the wavelength is small . In order to diffract the aperture must be similar in size to the object ( nothing with a gap of this order for us to interact with )
51
What is electron diffraction?
- Electrons are accelerated at high speeds in an evacuated tube and then passed through a thing graphite crystal sheet. - as the electrons pass through the gaps in the atoms of the crystal they diffract producing a diffraction pattern of circular rings
52
What is the diffraction pattern produced if you have a low accelerating voltage ?
Slower moving electrons therefore we get widely spaced rings
53
What is the diffraction pattern produced if there was a high accelerating voltage ?
Faster electrons meaning the rings would be closer together
54
What would you have seen if the electron beam behaved like a particle ?
- produce a spot / patch of light
55
How does the pattern produced on the screen ( electron diffraction ) support the idea that the electron beam is behaving as a wave ?
Wave properties - diffraction Graphite causes the electron waves to spread out - bright rings - constructive interference

Aqa Physics (16 decks)

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