The photoelectric effect

Another experiment which provides compelling proof for the photon nature of light is the photoelectric effect. In this effect light is shone at a metal plate, and it is found that electrons are ejected. These electrons then get accelerated to a nearby plate by an external potential difference, and a photoelectric current is established, as below.

 

Figure 12.9: Photoelectric effect

The explanation, which was first given by Einstein and which won him the Nobel Prize, is as follows. The photons hit an electron in the metal, giving up its energy, This is enough to free the electron from the attractive forces holding it in the metal, and it is accelerated towards the other side, causing a flow of charges and hence a current. This effect, which arises in devices such as automatic door openers, burglar alarms, light detectors, and photocopiers, cannot be explained using a wave picture of light. For example, it is found experimentally that the photoelectric current depends critically on the frequency of the light being used. This is a feature of the energy that the electrons gain when struck by the light, but in the wave picture the energy of the light depends on the amplitude, and not on the frequency. However, in the photon picture of light the energy of the photon is proportional to the frequency of the associated wave, which therefore provides a natural explanation of the frequency dependence of the photoelectric current.