The Problem of Reflection in Eighteenth-Century Projectile Theories of Light
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Physics in Perspective
The Problem of Reflection in EighteenthCentury Projectile Theories of Light Breno Arsioli Moura* This paper explores the mechanical models elaborated by projectile theorists throughout the eighteenth century to explain the reflection of light. Influenced by Isaac Newton’s Opticks, these projectile theorists proposed that repulsion was the cause of reflection. My purpose is to show that their models were not unified and lacked a deeper understanding of the origin of repulsive powers. This analysis illustrates how a simple optical phenomenon was not easy for eighteenth-century theorists to explain, even when the projectile theory of light was prominent among natural philosophers.
Key words: Reflection of light; repulsion; optics; Isaac Newton; eighteenth century.
Introduction Alongside refraction, the reflection of light is one of the most basic optical phenomena. Reflection follows a simple law: when a ray of light reaches the surface of a mirror or refracting substance, it is reflected at an angle equal to the angle of incidence. This law of reflection has been known since ancient times. With the advancement of the study of light in Europe in the seventeenth century, it became one of the pillars of geometrical optics. Although we can easily apply this law to explain the reflective behavior of light, it leaves open an important question: What is the cause of reflection? For projectile theorists in the eighteenth century, the answer to this question was not simple. They believed that light was composed of small particles emitted from luminous bodies. Moreover, they thought that these particles were subject to the same laws of dynamics as common bodies. These theorists were greatly influenced by Opticks (1704), Isaac Newton’s (1642–1727) major book on light and colors. The book discussed several subjects in this area, such as refraction and reflection, the color of bodies, colored rings in thin and thick films, and the ‘‘inflection’’ of light—diffraction, in current terms. Regarding the nature of light, * Breno Arsioli Moura is Assistant Professor of History of Science and Physics Teaching at Federal University of ABC, Brazil. His main research interests are in history of optics and eighteenth-century natural philosophy.
B. Arsioli Moura
Phys. Perspect.
Newton was more cautious. Although it is clear that he favored a corpuscular conception of light, he never openly defended it, only discussing this theory in the most speculative part of Opticks: the ‘‘Queries’’ at the end of book 3.1 In the ‘‘Queries,’’ Newton not only discussed the corpuscular nature of light but also speculated on its interaction with bodies through short-range forces of attraction and repulsion, causing refraction and reflection.2 In contrast to Newton, the projectile theorists were not cautious about the corpuscular nature of light. From the publication of Opticks in 1704 to the end of the century, their works explicitly discussed a projectile theory of light. These works sped the acceptance and adoption of the theory
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