The Particle Nature of Light (HL) (HL IB Physics)

• The photoelectric effect can be observed on a gold leaf electroscope

• A plate of metal, usually zinc, is attached to a rod and a strip of gold leaf
  • The plate is given a negative charge, causing the gold leaf to be repelled

• UV light is then shone onto the metal plate, leading to the emission of photoelectrons
• The gold leaf begins to fall back towards the central rod
  • This is because the plate, rod and gold leaf become less negatively charged and therefore repel less

Typical set-up of the gold leaf electroscope experiment

• Some notable observations from the gold leaf experiment:
  • The gold leaf begins to fall down instantly when illuminated with UV light
  • The gold leaf takes longer to fall at low intensities of UV light, although it still happens instantly
  • The gold leaf does not fall down when lower frequencies of light are used
  • The gold leaf does not fall down at lower frequencies, even if the intensity of the light is increased

• The photoelectric effect is evidence for the particle nature of light
• The observations cannot be explained by the wave model of light
  • The wave model predicts that photoelectrons would be released at any frequency since energy would be accumulated by the electron with each wavefront
  • Wave theory suggests that for a given frequency, the energy of the wave is proportional to the intensity of the beam of light; therefore, the kinetic energy of the emitted photoelectrons should be dependent on the intensity

Explanations of Photoelectric Emission

1. The gold leaf begins to fall down instantly when illuminated with UV light

• The fact that the gold leaf falls down at all indicates that electrons are emitted from the surface of the metal plate 
  • As electrons are emitted, the net charge on the plate and gold leaf decreases
  • The gold leaf is repelled less, so it falls down
• UV light has a high frequency, so the energy of a UV photon is sufficient to release an electron from the surface of the  metal
  • This supports a threshold frequency and minimum energy required to release an electron from the surface of the metal (work function)

2. The gold leaf falls slower at lower intensities of UV light, although it still happens instantly

• The fact that the gold leaf still falls indicates that electrons are still emitted at low intensities
• The fact that the gold leaf falls more slowly at lower intensities indicates that the intensity does affect the rate of photoelectric emission  
• The fact that electrons are released instantly supports the one-to-one quantised energy transfer from photon to electron 

3. The gold leaf does not fall down when lower frequencies of light are used

• The fact that the gold leaf does not fall when lower frequencies (for example, visible light) are used indicates that no electrons are emitted at lower frequencies
  • Lower frequency photons have lower energies so they do not have sufficient energy to release an electron from the surface of the metal
  • This supports the threshold frequency and minimum energy required to release an electron (work function)
  • This also indicates that it is intensity and not frequency that affects the rate of photoelectric emission

4. The gold leaf does not fall down at lower frequencies, even if the intensity of the light is increased

• The fact that the gold leaf still does not fall if the intensity increases indicates that no electrons are emitted below a certain frequency
• Higher intensity means more photons are incident, but none of them have sufficient energy to release an electron from the surface of the metal
  • This supports the threshold frequency and minimum energy required to free an electron from the surface (work function)

In the photoelectric effect, a single photon may cause a surface electron to be released if it has sufficient energy