The Photoelectric Effect
Physics: AQA A Level, topic 2.2.1
Photoelectric Effect
When light is shone on to a metal plate, electrons can be emitted. The conditions that determine whether electrons are emitted or not tell us a lot about the nature of light itself.
Threshold frequency
- Electrons are only emitted from a metal when the light hitting it is above a certain frequency.
- Below this frequency, no electrons are emitted.
- This frequency is called the threshold frequency.
- Threshold frequency is different for different types of metal but is usually in the UV range.
Kinetic energy variation
- The maximum kinetic energy of an emitted electron is dependent on the frequency of radiation but not its intensity (energy transferred per second).
Above the threshold frequency
- When the light (EM radiation) is above the threshold frequency of the metal, the number of electrons emitted is proportional to the intensity of the light.
Failure of Wave Theory
Wave theory wasn't able to account for the photoelectric effect.
Threshold frequency
- According to the wave theory of light, energy should be spread out evenly across the surface of the metal.
- The energy absorbed by each electron would gradually increase so that after a while the electrons would have enough energy to be emitted.
Intensity independence
- According to the wave theory of light, the intensity should determine the energy transferred to the electrons each second.
- So we would expect that the maximum kinetic energy of emitted electrons would increase with intensity (not frequency).
Test yourself on The Photoelectric Effect
Which of these statements about the photoelectric effect are true?
When the frequency of light is above the threshold frequency how is the number of photoelectrons related to the intensity of light?
What could wave theory not explain?
Explain why the existence of a threshold frequency in the photoelectric effect provided evidence that light was not a wave.
Explain why the maximum kinetic energy of emitted electrons being independent of intensity in the photoelectric effect provided evidence that light was not a wave.