Model Answers - 5c Wave Particle Duality

Question 1

Define intensity of light in the wave model of light

Answer 1

Intensity is power/area
Intensity is proportional to the amplitude of the wave squared - this is related to the energy of the wave

Question 2

define photon

Answer 2

discrete packet of electromagnetic energy

Question 3

describe what is meant by the wave particle duality of light

Answer 3

• light exhibits wave behaviour (diffraction, interference, superposition, polarisation)
• light exhibits particle behaviour (absorption and emission line spectra, photoelectric effect)
• so light exhibits both wave and particle behaviour: it exhibits wave particle duality

Question 4

define work function

Answer 4

minimum energy required for electron to be released from the surface of a metal

Question 5

Define threshold frequency

Answer 5

Minimum photon freqeuncy required for electron to be released from the surface of a metal (by absorbing the photon)

Question 6

Explain how the equation hf = Φ + KEmax is a conservation of energy equation

Answer 6

• hf is the photon energy
• when the electron absorbs the photon, the photon energy is transferred to the electron
• the electron leaves the surface of the metal using a minimum amount of energy called the work function Φ
• the remaining energy is transferred into the kinetic energy of the electron
• which will have a maximum value of KEmax when the electron has only used the work function to leave the surface of

Question 7

7. Explain why the kinetic energy of the electron in the equation hf = Φ + KEmax is the maximum kinetic energy

Answer 7

• the work function is the minimum energy required for electron to be released
• thus in the equation hf = work function + ke max, the kinetic energy is the maximum possible kinetic energy
• most electrons will have less kinetic energy than this as they require more energy in order to leave the surface of the metal (due to needing more than the minimum energy to overcome the electrostatic forces of attraction).

Question 8

8. Plot a graph of maximum kinetic energy of released electron against frequency of photon

Answer 8

A graph of energy (Joules) against frequency (Hz)
- a straight line
- with a negative y intercept
- gradient equal to h
- x intercept equal to threshold frequency (f0)
- y intercept = work function is

Question 9

6. Explain how the equation hf = Φ + KEmax is a conservation of energy equation

Answer 9

• hf is the photon energy
• when the electron absorbs the photon, the photon energy is transferred to the electron
• the electron leaves the surface of the metal using a minimum amount of energy called the work function Φ
• the remaining energy is transferred into the kinetic energy of the electron
• which will have a maximum value of KEmax when the electron has only used the work function to leave the surface

Question 10

Derive the equation relating threshold freqeuncy and work function

Answer 10

• The threshold frequency is the photon frequency that would cause electrons to just leave the metal surface but have no kinetic energy on escaping
• the equation hf = Φ + KEmax becomes
• hf0 = Φ where f0 is the work function
• so f0 = Φ / h

Question 11

Complete the following table describing the photoelectric effect observations and conclusions

Observation from the photoelectric effect What would be expected by the wave model Why this observation leads to the particle model of light

Answer 13

A graph of energy (Joules) against frequency (Hz)
- a straight line
- with a negative y intercept
- gradient equal to h
- x intercept equal to threshold frequency (f0)
- y intercept = work function