ABSTRACT
That light should have mechanical properties has been known, or at least suspected, since Kepler proposed that the tails of comets were due to radiation pressure associated with light from the sun. A quantitative theory of such effects became possible only after the development of Maxwell’s unified theory of electricity, magnetism and optics. However, although his treatise on electromagnetism (1.1) contains a calculation of the radiation pressure at the earth’s surface, there is little more on the mechanical effects of light. It was Poynting who quantified the momentum and energy flux associated with an electromagnetic field (1.2). In modern terms, the momentum per unit volume associated with an electromagnetic wave is given by "0E B. The angular momentum density is, naturally enough, the cross product of this with position, that is r "0ðE BÞ (1.3).
