The photoelectric effect is a natural phenomenon in which electrons are emitted by a metallic surface when it is exposed to electromagnetic radiation. It forms the basis for modern solar cells.
The photoelectric effect was first observed by Hertz when he was investigating the transmission of radio waves. He found that shining ultraviolet light onto the receiver increased the intensity of the spark produced. This was further investigated by von Lenard, who made a series of observations, some of which could not be explained by the classical wave theory of light:
- Electrons were only emitted above a certain frequency and that this emission was spontaneous, classical theory stated that they would absorb the energy until they could leave the atom
- The kinetic energy of the electrons (found by applying a stopping voltage) was dependent only on frequency, classical theory stated that this would be also affected by intensity
- The number of electrons was dependent on intensity and not frequency
The modern theory of the photoelectric effect is based in the quantum theory proposed by Planck. In 1900, he published the idea that the energy emitted by a resonator could only take on discrete values or quanta. The energy for a resonator of frequency v is hv where h is a universal constant, now called Planck's constant. In 1902, Lenard showed that the energy of electrons in the photoelectric effect depended only on the frequency of the incident light. In 1905, Einstein showed that Planck's formula could be used to give a heuristic explanation of Lenard's results. The phenomenon was thus explained as such:
- Each photon contained energy based solely on frequency, such that E = hf where h is Planck's constant (6.626 x 10-34)
- Electrons could be emitted if they received sufficient energy from a photon. However, this was an "all or nothing" transfer and emission would only occur if the single photon contained enough energy to overcome the work function of the material. The minimum frequency to cause emission is known as the threshold frequency
- The kinetic energy of each photon is the difference between the energy carried by the photon and the work function of the material
- The intensity of light was related to the number of photons, hence a greater intensity meant more change of collision between a photon and an electron and hence more electrons being emitted
Lenard, Planck, and Einstein won the Nobel Prize in physics in 1905, 1918 and 1921 respectively. Lenard was cited "for his work on cathode rays," Planck was cited "in recognition of the services he rendered to the advancement of Physics by his discovery of energy quanta" and Einstein's was cited for his "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect."