A metal contains conduction electrons, these move about freely, colliding with each other and with positive ions. If electromagnetic radiation above a certain frequency is directed at the surface of a metal, electrons will be emitted. For most metals this frequency is in the U.V. range.
1) Free electrons on the surface of the metal absorb energy from light which makes them vibrate.
2) If an electron absorbs enough energy, the bonds holding it to the metal surface break and it is released.
3) This is known as the photoelectric effect and any electrons emitted are called photoelectrons.
- For a given metal, no photoelectrons are emitted if the radiation has a frequency below a certain value -called the threshold frequency.
- Photoelectrons are emitted with a variety of different kinetic energies ranging from zero to a maximum value. This value of maximum kinetic energy increases with the frequency of radiation -it is unaffected by intensity of the radiation.
- The number of photoelectrons emitted per second is proportional to the intensity of the radiation.
Wave theory and the photoelectric effect
Wave theory cannot explain the photoelectric effect. According to wave theory;
1) For a particular frequency of light, energy carried is proportional to intesity of the beam.
2) Energy carried by light is spread evenly over the wavefront.
3) Each free electron on the surface of the metal would gain some energy from each incoming wave.
4) Gradually, each electron would gain enough energy to leave the metal.
Therefore, if light had a lower frequency (i.e. carrying less energy) it would take longer for the electrons to gain enough energy -but this would happen eventually. There's no explanation given for threshold frequency.
The higher the intensity of the wave, the more energy it should transfer to each electron -kinetic energy should increase with intensity. There's no explanation for kinetic energy depending only on frequency.
Einstein's photon model
When Max Planck was investigating black body radiation, he suggested EM waves can only be released in discrete packets, or quanta. The energy carried by these wave-packets had to be;
E = hf = hc/wavelength