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 gold leaf, which initially has a negative charge, causing it to be repelled by a central negatively charged rod
  • This causes negative charge, or electrons, to build up on the zinc plate

• UV light is shone onto the metal plate, leading to the emission of photoelectrons
• This causes the extra electrons on the central rod and gold leaf to be removed, so, the gold leaf begins to fall back towards the central rod
  • This is because they become less negatively charged, and hence repel less

• Some notable observations:
  • Placing the UV light source closer to the metal plate causes the gold leaf to fall at a faster rate
  • Using a light source of a higher frequency does not affect the rate at which the gold leaf falls
  • Using a filament light source causes no change in the gold leaf’s position
  • Using a positively charged plate also causes no change in the gold leaf’s position

Typical set-up of the gold leaf electroscope experiment

Laws of Photoelectric Emission

• The photoelectric effect is evidence for the particle nature of light, as demonstrated by the following observations

1. Placing the UV light source closer to the metal plate causes the gold leaf to fall more quickly

• Placing the UV source closer to the plate increases the intensity incident on the surface of the metal
• Increasing the intensity, or brightness, of the incident radiation increases the number of photoelectrons emitted per second
• Therefore, the gold leaf loses negative charge more rapidly

2. Using a higher frequency light source does not change how quickly the gold leaf falls

• The maximum kinetic energy of the emitted electrons increases with the frequency of the incident radiation
• In the case of the photoelectric effect, energy and frequency are independent of the intensity of the radiation
• So, the intensity of the incident radiation affects how quickly the gold leaf falls, not the frequency

3. Using a filament light source causes no change in the gold leaf’s position

• If the incident frequency is below a certain threshold frequency, no electrons are emitted, no matter the intensity of the radiation
• A filament light source has a frequency below the threshold frequency of the metal, so, no photoelectrons are released

4. Using a positively charged plate causes no change in the gold leaf’s position

• If the plate is positively charged, that means there is an excess of positive charge on the surface of the metal plate
• Electrons are negatively charged, so they will not be emitted unless they are on the surface of the metal
• Any electrons emitted will be attracted back by positive charges on the surface of the metal

5. Emission of photoelectrons happens as soon as the radiation is incident on the surface of the metal

• A single photon interacts with a single electron
• If the energy of the photon is equal to the work function of the metal, photoelectrons will be released instantaneously

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