Descartes’ work on optics spanned his entire career and represents a fascinating area of inquiry. His interest in the study of light is already on display in an intriguing study of refraction from his early notebook, known as Private Thoughts, dating from 1619 to 1621 (AT X 242–43). Optics figures centrally in Descartes’ The World (or Treatise on Light), written between 1629 and 1633, as well as, of course, in his Dioptrics (1637). It also, however, plays important roles in the three essays published together with the Dioptrics, namely, the Discourse on Method, the Geometry, and the Meteors, and many of Descartes’ conclusions concerning light from these earlier works persist with little substantive modification into the Principles of Philosophy (1644). In what follows, we look in a brief and general way at Descartes’ understanding of light, his derivations of the two central laws of geometrical optics, and a sampling of the optical phenomena he sought to explain. We will conclude by noting a few of the many ways in which Descartes’ efforts in optics prompted – through both agreement and dissent – further developments in the history of optics.

Descartes was a famously systematic philosopher, and his thinking about optics is deeply enmeshed with his more general mechanistic physics and cosmology. In the sixth chapter of The Treatise on Light, he asks his readers to imagine a new world “very easy to know, but nevertheless similar to ours,” consisting of an indefinite space filled everywhere with “real, perfectly solid” matter, divisible “into as many parts and shapes as we can imagine” (AT XI 9, G 21, n. 40) (AT XI 33–34, G 22–23). Of this world he postulates that “from the first instant of creation,” God “causes some [parts] to start moving in one direction and others in another, some faster and others slower” and that subsequently “he causes them to continue moving thereafter in accordance with the ordinary laws of nature” (AT XI 34, G 23). The laws of nature, Descartes suggests, in turn sort the world into three elements: a maximally fine element of fire, a coarser yet still “very subtle” element of air, and a relatively gross element of earth “whose parts have little or no motion” relative to one another (AT XI 24–25; G 17–18).