Quantum (28)

Question 1

Explain what is meant by a photon

Answer 1

A quantum of electromagnetic energy

Question 2

What is the equation for photon energy?

Answer 2

E = hf

where h = Planck’s constant, 6.63 x 10^-34

Question 3

What is meant by the threshold frequency?

Answer 3

The minimum frequency of the incident EM radiation that would eject electrons from the surface of a metal

Question 4

What is meant by the threshold wavelength?

Answer 4

The longest wavelength of the incident EM radiation that would eject electrons from the surface of a metal

Question 5

Define the work function

Answer 5

• The minimum photon energy required to electron an electron from the surface of a metal

Question 6

State which phenomenon provides evidence for the particulate nature of electromagentic radiation

Answer 6

The photoelectric effect

Question 7

State which phenomenon provides evidence for the wave nature of matter

Answer 7

Diffraction

Question 8

What is the de Broglie wavelength?

Answer 8

The wavelength associated with a moving particle

Question 9

What is the de Broglie wavelength equation?

Answer 9

λ=h/p

where p = momentum

Question 10

Explain why the KEmax of photoelectrons is independent of the intensity of the incident radiation

Answer 10

• Frequency determines the energy of a photon
• Intensity is only the power incident on the surface per unit area
• Thus, intensity does not determine the energy of a photon
• The kinetic energy of an electron depends on the frequency and thus, the energy of one photon

Question 11

Explain why photoelectric current is proportional to intensity

Answer 11

• If the frequency of photons is fixed and the energy is equal to or above the work function, increasing the intensity increases the number of (sufficient energy) protons hitting the metal surface per second
• As there is a one-to-one interaction, this means the number of photoelectrons released per second increases
• Thus, there is a greater charge per second and thus, a higher current

Question 12

State the effects if the intensity of a photon is kept constant but the frequency is increased

Assume the photon beam is already causing photoelectric emission

Answer 12

• If the frequency is increased, the photons contain a greater share of the total energy
• Thus, since there are fewer, yet higher energy photons hitting the metal surface, the rate of electron emission decreases
• However the KE increases aswell as the energy of the photons and emitted electrons

Question 13

Explain the formation of emission line spectra

Answer 13

• Electrons in atoms occupy discrete energy levels
• When energy is supplied (by heating etc) the electrons are excited to higher energy levels
• These electrons are unstable and de-excite to a lower energy level
• When doing this, they release photons with energy equal to the difference in energy levels
• As energy levels are fixed, only photons of discrete frequencies are emitted, thus discrete lines

Question 14

Explain the formation of absorption line spectra

Answer 14

• Light with a continuous wavelength passes through a gas
• Atoms in the gas absorb photons, which only occurs if the photon energy matches the gap between energy levels
• Electrons are excited to higher energy levels
• These absorbed wavelengths are missing from the transmitted light
  Thus, they appear as discrete dark lines