1. Context

The photons in a light beam have both momentum and energy. It is possible to convert light into mechanical work, by exchanging both momentum and energy. The transfer of angular momentum, which is characterized by radiation pressure, is exploited by only a few special systems (mainly solar sails and optical tweezers). The efficiency of this type of conversion remains low, and it is mainly through the transfer of light energy that an efficient exchange can exist and be truly exploited. This was first used empirically and then, since the beginning of the 20th century, understood and progressively applied in the conversion of radiation into thermal, electrical, chemical and electrochemical energy, particularly in the use of photovoltaic cells, batteries or biofuels. As all these energies can be converted into mechanical energy, there are many ways to obtain movement from light.