For many years light nature was one of controversial issues in physics. Some researchers, starting with I. Newton, represented light as a stream of particles (corpuscular theory), others adhered to the wave theory. But any of these theories separately did not explain all properties of light.
At the beginning of 20 century there is especially obvious a contradiction between the classical wave theory of light and results of experiments. In particular, it concerned photoeffect which is that substance under the influence of electromagnetic radiation – in particular, light – is capable to let out electrons. On it A. Einstein, as well as pointed to ability of substance to be in thermodynamic balance with a radiation.
The great value is gained at the same time by the idea of quantization of electromagnetic radiation (i.e. adoptions by it only of certain value, an indivisible portion – quantum) – contrary to the wave theory assuming that energy of electromagnetic radiation can be any.
Background of experience to Botha
Idea of the quantum nature of electromagnetic radiation in general and light in particular not at once it was accepted by all physics. Some of them explained quantization of energy at absorption and light radiations with properties of substances which absorb or radiate light. It could be explained with atom model with discrete levels of energy – such models were developed by A. Sommerfeld, N. Bohr. The experiment with a x-ray radiation conducted in 1923 by the American erudite A. Compton became a turning point. In this experience the dispersion of light quanta on free electrons called Compton effect was revealed. At that time was considered that the electron has no internal structure, therefore, it can have no energy levels. Thus, Compton effect proved the quantum nature of light radiation.
Experience Boat
In 1925 the next experiment proving the quantum nature of light, more precisely – quantization at its absorption was made. The German physicist Walter Bot conducted this experiment. The bunch of x-ray radiation of low intensity influenced a thin plate of a foil. At the same time there was a phenomenon of x-ray fluorescence, i.e. the foil itself began to emit weak x-ray radiation. These beams were fixed by two gas-discharge counters which were placed at the left and to the right of a plate. By means of the special mechanism of meter readings were fixed on a paper tape. In terms of the wave theory of light, the energy radiated by a foil had to be distributed evenly in all directions including in where there were counters. Marks in a paper tape at the same time would appear synchronously – one precisely opposite to another, but it did not occur: the chaotic arrangement of marks spoke about emergence of particles which flew in one, in other direction from a foil. Thus, experience Bot proved the quantum nature of electromagnetic radiation. Later electromagnetic quanta were called photons.