Demonstrating the Photoelectric Effect
- 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
Observations of the Gold Leaf Experiment
- Placing the UV light source closer to the metal plate causes the gold leaf to fall more quickly
- Using a higher frequency light source does not change how quickly the gold leaf falls
- Using a filament light source causes no change in the gold leaf’s position
- Using a positively charged plate causes no change in the gold leaf’s position
- Emission of photoelectrons happens as soon as the radiation is incident on the surface of the metal
Typical set-up of the gold leaf electroscope experiment
Explaining the Observations
- Observation:
Placing the UV light source closer to the metal plate causes the gold leaf to fall more quickly
- Explanation:
- 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
- Observation:
Using a higher frequency light source does not change how quickly the gold leaf falls
- Explanation:
- 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
- Observation:
Using a filament light source causes no change in the gold leaf’s position
- Explanation:
- 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
- Observation:
Using a positively charged plate causes no change in the gold leaf’s position
- Explanation:
- 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
- Observation:
Emission of photoelectrons happens as soon as the radiation is incident on the surface of the metal
- Explanation:
- 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
