Optical helicity of interfering waves

Helicity is a property of light which is familiar from particle physics but less well-known in optics. In this paper we recall the explicit form taken by the helicity of light within classical electromagnetic theory and reflect upon some of its remarkable characteristics. The helicity of light is related to, but is distinct from, the spin of light. To emphasise this fact, we draw a simple analogy between the <i>helicity</i> of light and electric <i>charge</i> and between the <i>spin</i> of light and electric <i>current</i>. We illustrate this and other observations by examining various superpositions of plane waves explicitly.