05b Nature Of Light

Question 1

What is diffraction?

Answer 1

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

Question 2

What criteria must be met for maximum diffraction to occur

Answer 2

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

Question 3

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

Answer 3

The wave will be reflected.

Question 4

State the diffraction grating equation.

Answer 4

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

Question 5

What does electron diffraction provide evidence for?

Answer 5

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

Question 6

Describe the diffraction pattern produced by electrons.

Answer 6

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

Question 7

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

Answer 7

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

Question 8

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

Answer 8

De Broglie wavelength.

Question 9

What two factors does the de Broglie wavelength depend on?

Answer 9

1. Mass
2. Velocity

Question 10

State the equation used to calculate a de Broglie wavelength.

Answer 10

λ = h/mv
H is Planck’s constant

Question 11

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

Answer 11

p, momentum

Question 12

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

Answer 12

• 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)

Question 13

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

Answer 13

1. The wavelength of the radiation
2. The duration of the pulse

Question 14

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

Answer 14

1. The wave model
2. The particle model

Question 15

Which model does the photoelectric effect provide evidence for?

Answer 15

The particle model.

Question 16

Outline the photoelectric effect.

Answer 16

• 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

Question 17

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

Answer 17

Photons

Question 18

How do you calculate the energy of a photon?

Answer 18

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

Question 19

Explain how a photon can liberate an electron.

Answer 19

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.

Question 20

What is threshold frequency?

Answer 20

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

Question 21

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

Answer 21

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

Question 22

Why are photoelectrons emitted with a range of kinetic energies?

Answer 22

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.

Question 23

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

Answer 23

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

Question 24

What is the conversion factor between eV and J?

Answer 24

1eV = 1.6 × 10-19 J

Question 25

What happens when electrons transition between energy levels?

Answer 25

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

Question 26

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

Answer 26

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.

Question 27

What is a wave front

Answer 27

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

Question 28

Electron volt definition

Answer 28

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

Question 29

Work done and charge equation

Answer 29

W = QV

Question 30

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

Answer 30

E = hf
E= hc / λ

C = speed of light, h = Plancks constant

Question 31

What does discrete energy level mean?

Answer 31

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

Question 32

What happens after the excitation of an electron?

Answer 32

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

Question 33

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

Answer 33

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

Question 34

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

Answer 34

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

Question 35

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

Answer 35

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

Question 36

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

Answer 36

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

Question 37

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

Answer 37

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

Question 38

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

Answer 38

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.

Question 39

Why do excited atoms only emit certain frequencies?

Answer 39

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

Question 40

What is the lyman series

Answer 40

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

Question 41

What is the Balmer series

Answer 41

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

Question 42

Explain the stopping voltage experiment

Answer 42

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.

Question 43

Explain the double slit experiment

Answer 43

When electrons or 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.

Question 44

Explain the uv catastrophe

Answer 44

When the energy gets so high the frequency drops to 0

Question 45

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

Answer 45

• 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).

Question 46

Explain the emission spectrum and absorption spectrum and what it shows

Answer 46

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.

Question 47

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

Answer 47

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).