Opticks: or, A Treatise of the Reflexions, Refractions, Inflexions and Colours of Light is a collection of three books by Isaac Newton that was published in English in 1704 (a scholarly Latin translation appeared in 1706).

The publication of Opticks represented a major contribution to science, different from but in some ways rivalling the Principia, yet Isaac Newton's name did not appear on the cover page of the first edition.

Opticks is largely a record of experiments and the deductions made from them, covering a wide range of topics in what was later to be known as physical optics.

Newton sets forth in full his experiments, first reported to the Royal Society of London in 1672,[2] on dispersion, or the separation of light into a spectrum of its component colours.

He demonstrates how the appearance of color arises from selective absorption, reflection, or transmission of the various component parts of the incident light.

In an Experimentum crucis or "critical experiment" (Book I, Part II, Theorem ii), Newton showed that the color of light corresponded to its "degree of refrangibility" (angle of refraction), and that this angle cannot be changed by additional reflection or refraction or by passing the light through a coloured filter.

Unlike the Principia, which vowed Non fingo hypotheses or "I make no hypotheses" outside the deductive method, the Opticks develops conjectures about light that go beyond the experimental evidence: for example, that the physical behaviour of light was due its "corpuscular" nature as small particles, or that perceived colours were harmonically proportioned like the tones of a diatonic musical scale.

Stephen Hales, a firm Newtonian of the early eighteenth century, declared that this was Newton's way of explaining "by Quaere.

[9] This query predates the prediction of gravitational lensing by Albert Einstein's general relativity by two centuries and later confirmed by Eddington experiment in 1919.

Various 18th century historians and chemists like William Cullen and Torbern Bergman, credited Newton for the development affinity tables.

Newtonian science became a central issue in the assault waged by the philosophes in the Age of Enlightenment against a natural philosophy based on the authority of ancient Greek or Roman naturalists or on deductive reasoning from first principles (the method advocated by French philosopher René Descartes), rather than on the application of mathematical reasoning to experience or experiment.

Voltaire popularised Newtonian science, including the content of both the Principia and the Opticks, in his Elements de la philosophie de Newton (1738), and after about 1750 the combination of the experimental methods exemplified by the Opticks and the mathematical methods exemplified by the Principia were established as a unified and comprehensive model of Newtonian science.