The Particle Theory of Light

Abstract

what, according to Newton, is a ray of light? And why do rays of light obey the laws of optics which he laid out so succinctly in Book I of his Optics? The next two chapters are drawn from Newton’s Queries included in Book III of his Opticks. These Queries take the form of rhetorical questions by which Newton articulates his theory of light. He begins slowly, exploring the relationship between light, heat and matter. In particular, in the first 24 Queries Newton suggests that when a body vibrates—whether due to heat, friction, percussion, or chemical action—it emits light. Conversely, when light strikes a substance it initiates vibrations within the substance. For instance, when light strikes the eye, vibrations arise in the vitreous humor; these vibrations, in turn, cause the visual perception of color. Likewise, when light strikes a transparent medium—such as water, air or even æther—vibrations arise in the medium; these vibrations, in turn, affect the trajectory of the light within the medium. Indeed, far from rejecting the existence of æther, Newton actually argued that its existence (or rather, inhomogeneities in its density) are responsible for the bending of light rays. Newton, however, carefully distinguishes light itself from vibrations of the æther. This is critical to Newton: vibrations of the æther can certainly affect the trajectory of a ray of light, but light is not a vibration of the æther. We join the discussion at Query 25, where Newton begins to articulate the specific properties that a ray of light must have in order to explain the curious phenomenon of birefringence observed in Iceland crystal.