Snell formulates the law of refraction

Timeline

1. 1617–1621Leiden University · LeidenSnell succeeded his father to the Leiden mathematics chair, measured an arc of latitude in the Netherlands by triangulation, and studied Kepler's Ad Vitellionem Paralipomena and Risner's Optica. His practical experience with trigonometry in geodesy prepared the mathematical tools he later used to express refraction in terms of sines (or secants).
2. 1621Leiden · LeidenIn an unfinished optical manuscript Snell stated that the ratio of the sine of the angle of incidence to the sine of the angle of refraction equals the ratio of the refractive indices of the two media. This formulation became the fundamental law of modern geometric optics, enabling the calculation of lens imaging and the bending of light at an interface. That same year he also published De Circuli Mensura, but the law of refraction itself was never printed.
3. 1626Leiden · LeidenSnell died in Leiden on October 30, 1626, with his optical treatise still in manuscript. During his lifetime he had mentioned the law to Isaac Beeckman and others; the manuscript was later examined by Isaac Vossius and Christiaan Huygens, becoming the main evidence for the posthumous attribution.
4. 1637Leiden · LeidenIn 1637 Descartes published the sine form of the law of refraction in his Dioptrique without mentioning Snell. Because Descartes was closely associated with Beeckman, later scholars have endlessly debated whether he discovered the law independently or learned of it indirectly; in the English-speaking world it is still commonly called Snell's law, in memory of the unpublished breakthrough of 1621.