## Section 1. INTRODUCTION

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).