Photoelectric effect
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.
Observations
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
Modern Theory
The modern theory of the photoelectric effect is based in the quantum theory proposed by Planck. In 1905 Einstein applied this to the photoelectric effect, postulating that light was not a continuous wave but a series of discrete packets or quanta of energy distributed over a wavefront. This "discovery of the law of the photoelectric effect" (explaining the effect in terms of quantized energy and providing equations) was the reason the Nobel Prize Committee awarded Einstein the Nobel Prize in 1921.[1] 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