Christiaan Huygens (14.04.1629-8.07.1695) was a Dutch physicist, mechanician, mathematician and astronomer. He was born in The Hague. He studied at the universities of Leiden (1645-47) and Breda (1647-49). In 1665 — 81 he lived in Paris, where he was elected a member of the Paris Academy of Sciences, and from 1681 he was again in The Hague.
His research in physics was in the field of mechanics, optics and molecular physics. He designed the first pendulum clock with an escapement (1656), worked out its theory (1673) and a number of problems connected with it. In particular, he solved the problem of determining the centre of oscillation of a physical pendulum and its period of oscillation, and established the laws governing centripetal force. He also studied the collision of elastic bodies and derived its laws (1669), and established the laws of conservation of momentum and of “living” forces.
In 1678, in a memoir presented to the Paris Academy of Sciences, he developed the wave theory of light (published in “Treatise on Light” in 1690). In explaining the mechanism of the propagation of light he put forward the well-known principle that bears his name (Huygens's principle). Starting from his theory of light, he explained a number of optical phenomena. He also studied double refraction and established some of its regularities; he measured with great accuracy the geometrical characteristics of Iceland spar, in which the phenomenon was observed, and detected it in crystals of quartz. He introduced the concept of the “crystal axis”. In 1678 he discovered the polarisation of light.
Together with R. Hooke he established (1665) the fixed points of the thermometer — the melting point of ice and the boiling point of water. He showed (1667) that water expands on freezing. He worked on the improvement of the telescope, and of its objectives in particular; he designed an eyepiece (the Huygens eyepiece) that is still in use, and introduced diaphragms. With the telescope he had built he discovered in 1655 the ring of Saturn and Saturn's first satellite, Titan, and determined its period of revolution round the planet. He worked on the so-called planetary machine, which was a forerunner of the planetarium, and on the theory of the figure of the Earth. He came close to discovering the law of universal gravitation. He was the first to conclude that the Earth is flattened at the poles, and he put forward the idea of measuring the acceleration of gravity by means of a seconds pendulum. He achieved significant results in mathematics.