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 helicity of light and electric charge and between the spin of light and electric current. We illustrate this and other observations by examining various superpositions of plane waves explicitly.螺旋度（helicity）是光的一项物理属性，该概念在粒子物理学中广为熟知，却在光学领域相对鲜少受到关注。本文首先回顾经典电磁理论框架下光的螺旋度的显式表达式，并探讨其若干显著特性。光的螺旋度与光的自旋（spin）存在关联，但二者有着明确的本质区别。为突出这一区别，我们将光的螺旋度类比于电荷，将光的自旋类比于电流。我们通过对各类平面波叠加态的具体分析，对上述观点及其他相关观测结果进行了阐释。