The Dioptrique, often translated as the Optics or, more literally, as the Dioptrics is one of Descartes’ earliest works. Likely begun in the mid- to late 1620s, Descartes refers to it by name in a letter to Mersenne of November 25, 1630 (AT I 182, CSMK 29). Its subject matter partially overlaps with Descartes’ more foundational project The World (or Treatise on Light) in which he offers a general mechanistic account of the universe including the formation, transmission, and reception of light. Although Galileo's condemnation by the church prompted Descartes to abandon, in 1633, his plans for publishing The World, he continued in the ensuing years to vigorously pursue other scientific projects, including projects related to his work in optics. He was eventually persuaded to publish three essays highlighting some of his discoveries together with an introductory essay concerning “the method for rightly directing one's reason and searching for truth in the sciences” (AT VI 1, O 3). As one of those essays, Descartes’ Dioptrics finally appeared in print together with the Discourse on Method, the Meteors, and the Geometry in the summer of 1637 in a French-language edition. It was republished in a Latin edition (without the Geometry) in 1644.

The subject matter of the Dioptrics may be thought of as covering three main topics and is formally divided by Descartes into ten chapters or “discourses.” The first main topic concerns the nature of light and the laws of optics. In the first discourse, Descartes invites his readers to “consider light as nothing else … than a certain movement or action, very rapid and very lively, which passes toward our eyes through the medium of the air and other transparent bodies” (AT VI 84, O 67). In the second discourse, Descartes attempts to derive the law of reflection (known since antiquity) and the law of refraction (first published in the Dioptrics) through a series of ingenious, mechanistic analogies to the behavior of tennis balls reflecting off of hard surfaces and puncturing thin sheets of cloth (see analogy).

The second main topic of the Dioptrics concerns human vision.