Christiaan Huygens, Dutch mathematician, astronomer, and physicist (d. 1695)
Christiaan Huygens, Lord of Zuilichem, ( HY-gənz, also US: HOY-gənz, Dutch: [ˈkrɪstijaːn ˈɦœyɣə(n)s] (listen), also spelled Huyghens; Latin: Hugenius; 14 April 1629 – 8 July 1695) was a Dutch mathematician, physicist, astronomer and inventor, who is regarded as one of the greatest scientists of all time and a major figure in the scientific revolution. In physics, Huygens made groundbreaking contributions in optics and mechanics, while as an astronomer he is chiefly known for his studies of the rings of Saturn and the discovery of its moon Titan. As an inventor, he improved the design of telescopes and invented the pendulum clock, a breakthrough in timekeeping and the most accurate timekeeper for almost 300 years. An exceptionally talented mathematician and physicist, Huygens was the first to idealize a physical problem by a set of parameters then analyse it mathematically, and the first to fully mathematize a mechanistic explanation of an unobservable physical phenomenon. For these reasons, he has been called the first theoretical physicist and one of the founders of modern mathematical physics.In 1659, Huygens derived geometrically the now standard formulae in classical mechanics for the centripetal force and centrifugal force in his work De vi Centrifuga. Huygens also identified the correct laws of elastic collision for the first time in his work De Motu Corporum ex Percussione, published posthumously in 1703. In the field of optics, he is best known for his wave theory of light, which he proposed in 1678 and described in his Traité de la Lumière (1690). His mathematical theory of light was initially rejected in favour of Newton's corpuscular theory of light, until Augustin-Jean Fresnel adopted Huygens's principle to give a complete explanation of the rectilinear propagation and diffraction effects of light in 1821. Today this principle is known as the Huygens–Fresnel principle.
Huygens invented the pendulum clock in 1657, which he patented the same year. His research in horology resulted in an extensive analysis of the pendulum in Horologium Oscillatorium (1673), regarded as one of the most important 17th century works on mechanics. While the first and last parts contain descriptions of clock designs, most of the book is an analysis of pendulum motion and a theory of curves. In 1655, Huygens began grinding lenses with his brother Constantijn to build telescopes for astronomical research. He was the first to identify the rings of Saturn as "a thin, flat ring, nowhere touching, and inclined to the ecliptic," and discovered the first of Saturn's moons, Titan, using a refracting telescope. In 1662 Huygens developed what is now called the Huygenian eyepiece, a telescope with two lenses, which diminished the amount of dispersion.
As a mathematician, Huygens developed the theory of evolutes and wrote on games of chance and the problem of points in Van Rekeningh in Spelen van Gluck, which Frans van Schooten translated and published as De Ratiociniis in Ludo Aleae (1657). The use of expectation values by Huygens and others would later inspire Jacob Bernoulli's work on probability theory.