Huygens invented the pendulum clock in 1657, which he patented the same year. Today this principle is known as the Huygens–Fresnel principle. 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. In 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). In 1659, Huygens derived geometrically the standard formulae in classical mechanics for the centrifugal force in his work ''De vi Centrifuga'', a decade before Newton. Huygens first identified the correct laws of elastic collision in his work ''De Motu Corporum ex Percussione'', completed in 1656 but published posthumously in 1703. For these reasons, he has been called the first theoretical physicist and one of the founders of modern mathematical physics. 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. As an engineer and 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. 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. Christiaan Huygens Christiaan Huygens, Lord of Zeelhem, (, ,, also spelled Huyghens 14 April 1629 – 8 July 1695) was a Dutch mathematician, physicist, engineer, astronomer, and inventor, who is regarded as one of the greatest scientists of all time and a major figure in the scientific revolution.