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A Level Physics > 05b Nature Of Light > Flashcards

05b Nature Of Light Flashcards

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1
Q

What is diffraction?

A

Diffraction is the spreading out of a wave as it passes through a gap.

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2
Q

What criteria must be met for maximum diffraction to occur

A

The size of the gap must be of the same magnitude as the wavelength of the wave.

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3
Q

What happens if the gap is much smaller than the wavelength of the wave?

A

The wave will be reflected.

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4
Q

State the diffraction grating equation.

A

nλ = dsinθ
n = order of maxima
d = the separation of each slit

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5
Q

What does electron diffraction provide evidence for?

A

The wave nature of electrons. It suggests that particles can demonstrate wavelike properties.

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6
Q

Describe the diffraction pattern produced by electrons.

A

Concentric circles of bright and dark fringes from a central bright point.

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7
Q

If electrons didn’t have a wave nature, describe the pattern that would be produced when they pass through a slit.

A

The electrons would be unaffected by the gap and pass straight through. A single bright region would be formed

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8
Q

What is the name given to the wavelength of a particle?

A

De Broglie wavelength.

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9
Q

What two factors does the de Broglie wavelength depend on?

A
  1. Mass
  2. Velocity
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10
Q

State the equation used to calculate a de Broglie wavelength.

A

λ = h/mv
H is Planck’s constant

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11
Q

What can ‘mv’ be replaced with in the de Broglie equation?

A

p, momentum

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12
Q

What is the basic process of a pulse-echo technique?

A

• A wave pulse is emitted
• It is transmitted and reflected at the boundary between two media
• The returning wave (echo) is detected
• The speed and time taken are used to calculate the distance to the object (divide the time by 2)

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13
Q

Suggest two things that may limit the amount of information that can be obtained by a pulse-echo technique.

A
  1. The wavelength of the radiation
  2. The duration of the pulse
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14
Q

What are the two models that can be used to describe electromagnetic radiation?

A
  1. The wave model
  2. The particle model
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15
Q

Which model does the photoelectric effect provide evidence for?

A

The particle model.

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16
Q

Outline the photoelectric effect.

A

• Light is shone on a metal plate
• If the light has a high enough frequency or energy is above the work function, electrons are emitted from the metal surface
• If the frequency is too low, no electrons are emitted hence the equation E=hf

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17
Q

What are the particles of light used to explain the photoelectric effect called?

A

Photons

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18
Q

How do you calculate the energy of a photon?

A

E = hf
h is Planck’s constant
fis the frequency of light

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19
Q

Explain how a photon can liberate an electron.

A

One photon interacts with one electron and transfers all its energy to it. If this energy is greater than the metal’s work function, the electron will have sufficient energy to be released.

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20
Q

What is threshold frequency?

A

A metal’s threshold frequency is the minimum frequency that a photon requires to liberate an electron from its surface.

21
Q

If the intensity of light being shone on a metal increases, how does the energy of the photoelectrons change?

A

The energy remains unaffected. An increase in intensity means more photons per area and so more photoelectrons are emitted.

22
Q

Why are photoelectrons emitted with a range of kinetic energies?

A

The electrons are at different depths in the metal/exist in discrete energy levels and so require different amounts of energy to be liberated. The excess energy from a photon once an electron has been liberated, is the kinetic energy of the electron.

23
Q

State the equation for the maximum kinetic energy of a photoelectron.

A

½ mv^2= hf - ϕ
ϕ is the metal’s work function

24
Q

What is the conversion factor between eV and J?

A

1eV = 1.6 × 10-19 J

25
Q

What happens when electrons transition between energy levels?

A

• If electrons move to a higher energy level, radiation must be absorbed
• If electrons move to a lower energy level, radiation is emitted

26
Q

Why can only certain frequencies of radiation be absorbed by an atom to cause an electron transition?

A

The electrons can only exist in discrete energy levels. The energy of the photon absorbed must be the exact amount of energy required to cover the difference between two discrete energy levels.

27
Q

What is a wave front

A

All the places affected by a certain wave in the same way at a point in time

28
Q

Electron volt definition

A

The eV is the amount of energy gained by one electron when it is accelerated through a potential difference of one volt

29
Q

Work done and charge equation

A

W = QV

30
Q

What are the two equations used for the energy of a photon of light

A

E = hf
E= hc / λ

C = speed of light, h = Plancks constant

31
Q

What does discrete energy level mean?

A

Electrons can early occupy discreet energy levels
Discreet means a whole energy level, such as 1,2,3 not 1.3, 4.8 etc

32
Q

What happens after the excitation of an electron?

A

Almost straight away after excitation, the election will de-excite back to a lower level. This will release EM energy
The bigger the drop back down, the more energy is released

33
Q

How does the photoelectric affect prove that light is a particle?

A

Frequency is a thing that matters when trying to liberate an electron from a metal plate, not intensity. For example, if visible light was shone onto the metal plate, even there was an extremely high intensity, and for a long period of time, it still wouldn’t liberate an electron. However, if the source is above the threshold frequency, one photon is absorbed by one electron and it will liberate an electron no matter the intensity. Electrons given enough energy to escape material are called photoelectrons. This proves E = hf as frequency is the influencing factor not intensity like the wave theory suggests.
If light was a wave, then high enough intensity light would liberate electrons, but this doesn’t happen. Its the frequency which matters. This is because light is a particle

34
Q

What should you say in every 5/6 marker about the photoelectric effect

A

One photon can only deliver to one electron OR one electron absorbs one photon
Each light particle carries energy, E = hf, and that energy must be high enough to get an electron free

35
Q

What does the gradient, x intercept and y intercept of a stopping voltage - frequency graph show

A

Gradient = h / e
X intercept = the threshold frequency
Y intercept = ϕ/ e

36
Q

What does the gradient, x intercept and y intercept of a kinetic energy - frequency graph show

A

Gradient = h (plancks constant)
X intercept = threshold frequency
Y intercept = ϕ (work function)

37
Q

What does the emission spectrum show and how do you get it?

A

The emission spectrum shows the various wavelength of light which are emitted when an electron is de-excited
For example, for helium. The certain wavelength of photons, which are given off by helium as it’s de-excited.
When an electron is de-excited, it emits a certain photon of a certain frequency
To see it, you shine, the white light through a diffraction grating to get them to split up

38
Q

What is to conclude from both the emission spectrum and the absorption spectrum

A

The frequencies which are absorbed by the electrons to move up the energy levels are the same as the photons given out when the electrons get the excited.

39
Q

Why do excited atoms only emit certain frequencies?

A

Photons are released when you go down an energy level
The energy of the photon = h f
e = the difference in energy levels
Electrons exist in discrete energy levels
Only certain energy changes are possible, so only certain frequencies are possible

40
Q

What is the lyman series

A

Electron transitions from higher energy levels to the n = 1 energy level. These transitions emit UV radiation

41
Q

What is the Balmer series

A

Electron transitions from higher energy levels to the n = 2 energy level. These transitions emit visible light.

42
Q

Explain the stopping voltage experiment

A

A photon is shone onto an anode (negative). One photon is absorbed by one electron. These electrons are repelled and attracted to the cathode causing a current to flow. The power supply is attached to the circuit. The current is adjusted with the potentiometer until the current equals zero. From this, you can work out the stopping voltage and therefore the maximum kinetic energy.

43
Q

Explain the double slit experiment

A

When electrons of light are shone through a double slit, they effectively act as both particles and waves at the same time. As the electrons/photons go through the double slits, they diffract. However, they land on the screen as particles would but they also produce multiple fringes like a wave would. So, despite them travelling like a wave, they also act as particles.

44
Q

Explain the uv catastrophe

A

When the energy gets so high the frequency drops to 0

45
Q

Describe the experiment used to get an absorption spectrum and an emission spectrum

A

• Shine the light from a black body radiator through a material (eg hydrogen gas).
• Separate the light out with a spectroscope or measure the wavelength intensities received with a digital meter.
• Observe the full EM Spectrum present EXCEPT a few black lines at specific wavelengths. These are ABSORPTION LINES (AL).
OR
• Excite a gas (eg mercury vapour, metal salt etc) with incident energy (eg electric current in a light tube, stick the salt in a bunsen burner flame etc.)
• Separate the light out with a spectroscope or measure the wavelength intensities received with a digital meter.
• Observe a few DISCRETE WAVELENGTHS of colour given off. These are EMISSION LINES (EL).

46
Q

Explain the emission spectrum and absorption spectrum and what it shows

A

The lines have wavelength that corresponds to the DIFFERENCE IN ENERGY LEVELS for the material they went through (AL) or were given off by (EL). Photon energy released (EL) or absorbed (AL) E=hf or E=hc / λ. For absorption lines, it represents an EXCITATION; and for emission, lines it represents a DE-EXCITATION.

47
Q

Explain what to do with the data from the stopping volatage experiment

A

Photoelectric Equation: hf=∅+E_Kmax.
The photocurrent is 0 when E_Kmax is 0 so there are no photoelectrons crossing the gap.
This happens when work is done against the electrons: E_Kmax=W.
We know that electrical work W=VQ so here E_Kmax=W=Ve where V is the stopping voltage.
Rearrange: E_Kmax=hf-∅,
Therefore: E_Kmax/e=hf/e-∅/e so V=hf/e-∅/e.
We can then EITHER:
Multiply the stopping voltage values by e to get E_Kmax in Joules, and then plot a graph of E_Kmax against frequency, where the x-intercept is the threshold frequency, the gradient is h and the y-intercept is ∅, so the magnitude of the y-intercept is the work function in Joules (J), OR
Plot a graph of stopping voltage against frequency, where the x-intercept is the threshold frequency, the gradient is h/e and the y-intercept is ∅/e, so the magnitude of the y-intercept is the work function in electron-Volts (eV).

48
Q

What is white light

A

Contains all colours so many different wavelengths within it

49
Q

Describe Huygens construction

A

A combination of wavelets that superpose/ interfere

A Level Physics (6 decks)

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