2.2 EM radiation and quantum phenomena

Question 1

What is threshold frequency (f₀)?

Answer 1

f₀ is the minimum photon frequency for an electron to be released from the surface.

Question 2

What is the Work function ϕ?

Answer 2

The minimum energy required by an electron to overcome the metallic bond

Question 3

What is the maximum KE of an photoelectron leaving a metal surface?

Answer 3

Question 4

What is the Stopping potential (V_s)?

Answer 4

The potential difference required to stop a photoelectron with KE_max.

Question 5

What is an electronvolt?

Answer 5

Kinetic energy carried by an electron after it has been accelerated from rest through a potential difference of 1 volt

Question 6

What are the conclusions made from the photoelectric effect?

Answer 6

• No photoelectrons emitted below threshold frequency
• Photoelectrons emitted with variety of kinetic energies depending on frequency of photon
• KE of photoelectrons is unaffected by intensity of photons
• Number of photoelectrons emitted per second is proportional to intensity of photons

Question 7

What does the photoelectric effect give evidence for?

Answer 7

Particle properties of light

• Wave theory cannot explain why intensity of photon doesn’t increase KE of photoelectrons released, but particle theory does.
• Energy increases with intensity in waves

Question 8

Define excitation.

Answer 8

When an atomic electron absorbs a specific amount of energy to jump to a higher energy level.

Question 9

Define ionisation.

Answer 9

When an atomic electron absorbs a enough energy to escape the atom.

Question 10

How does a fluorescent tube work?

Answer 10

• 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 its electrons are excited, they cascade down the energy levels and emit visible light photons.

Question 11

What is used as evidence for atoms having discrete energy levels?

Answer 11

Line spectra

Question 12

What is the energy emitted/absorbed between two energy levels?

Answer 12

Question 13

What is an emission spectrum?

Answer 13

• Bright lines on a black backgroud
• Each line corresponds to a specific wavelength
• Obtained from diffracting light

Question 14

What is an absorption spectrum?

Answer 14

• Black lines on a bright background
• Each line corresponds to a specific wavelength
• Obtained from passing white light through a cool gas

Photons of specific wavelengths are absorbed exciteing atomic electrons.

Question 15

What does electron diffraction suggest?

Answer 15

That electrons can behave as waves.

Question 16

What is wave-particle duality?

Answer 16

• Particles can behave like waves
• Waves can behave like particles

Question 17

How would the amount of diffraction change if a particle’s momentum was increased?

Answer 17

Increase in momentum leads to an decrease in wavelength, hence the amount diffraction decreases.

The maxima of the interference pattern become closer.