3.2.2/3.12.2.5 Electromagnetic Radiation And Quantum Phenomena

Question 1

What is the photoelectric effect?

Answer 1

Photoelectrons are emitted from the surface of a metal after light above a certain frequency is shone on it

Question 2

What is the threshold frequency?

Answer 2

The certain frequency for a type of metal that allows for the photoelectric effect to happen

Question 3

How can threshold frequency be explained by the photon model of light?

Answer 3

• EM waves travel in discrete packets called photons
• Each electron can absorb a single photon, therefore a photoelectron is only emitted if the frequency is above the threshold frequency
• If the intensity of the light is increased, and the frequency is above the threshold, more photoelectrons are emitted per second

Question 4

Why can’t threshold frequency be explained by wave theory?

Answer 4

The wave theory suggests that any frequency of light should be able to cause photoelectric emission as the energy absorbed by each electron will gradually increase with each incoming wave

Question 5

What is the work function of a metal?

Answer 5

The minimum energy required for electrons to be emitted from the surface of a metal

Question 6

What is the stopping potential?

Answer 6

The potential difference you would need to apply across the metal to stop the photoelectrons with the maximum kinetic energy
Measuring it allows you to find the maximum kinetic energy of the released photoelectrons

Question 7

What is ionisation?

Answer 7

Where electrons gain enough energy in an atom to be lost from it

Question 8

What is excitation?

Answer 8

When an electron doesn’t gain sufficient energy to be lost from the atom, so it is temporarily promoted to an energy level further from the nucleus
It will then fall back to its original energy level and release the energy in the form of a photon of EM energy

Question 9

Describe the fluorescent tube.

Answer 9

• Mercury vapour inside low pressure tube
• Mercury atoms are ionised (releasing more free electrons) and excited by colliding with free electrons
• Mercury atoms de-excite to release ultraviolet photons
• Fluorescent coating on inside of tube absorbs UV photons, exciting electrons in the coating, which de-excite to emit photons in the visible light spectrum

Question 10

What is an electron volt?

Answer 10

The energy gained by one electron when passing though a potential difference of 1 volt

Question 11

How do you convert from J to eV?

Answer 11

Divide by e (1.6x10^-19)

Question 12

How do you convert from eV to J?

Answer 12

Multiply by e (1.6x10^-19)

Question 13

What does each line in a line spectrum represent?

Answer 13

A different wavelength of light emitted

Question 14

Why is a line spectrum not continuous?

Answer 14

It only contains discrete values of wavelength

Question 15

What is the evidence for electrons in atoms only being able to transition between discrete energy levels?

Answer 15

Line spectra only show discrete values of wavelength, that the only photon energies emitted will correspond to

Question 16

What are examples of light acting as a wave?

Answer 16

Diffraction and interference

Question 17

What is an example of light acting as a particle?

Answer 17

The photoelectric effect

Question 18

How and why does the amount of diffraction change when the momentum of the particle is changed?

Answer 18

When momentum increases, wavelength decreases, and therefore the amount of diffraction decreases, so the concentric rings of the interference pattern become closer (λ=h/mv)

Question 19

What is the de Broglie hypothesis?

Answer 19

All particles have a wave-like nature and a particle nature
λ=h/mv

Question 20

What must happen in order for experimental evidence to be validated?

Answer 20

Peer review

Question 21

How does electron diffraction provide experimental evidence for the de Broglie hypothesis?

Answer 21

It showed that electrons (particles) can undergo diffraction, which can only be experienced by waves

Question 22

Describe the electron diffraction experiment.

Answer 22

• Electron gun accelerates electrons though a vacuum tube towards a crystal lattice
• Electrons interact with the small gaps between the atoms to form a diffraction pattern on a fluorescent screen behind the crystal

Question 23

What is the relationship between the accelerating voltage and the de Broglie wavelength of electrons?

Answer 23

As the accelerating voltage increases, the speed of electrons increases, decreasing their de Broglie wavelength, decreasing the fringe spacing, following wave theory