Robert Hooke (born July 18 [July 28, New Style], 1635,Freshwater,Isle of Wight, England—died March 3, 1703, London) was anEnglishphysicist who discovered thelaw ofelasticity, known asHooke’s law, and who did research in a remarkable variety of fields.

In 1655 Hooke was employed byRobert Boyle to construct the Boylean air pump. Five years later, Hooke discovered his law of elasticity, which states that the stretching of a solid body (e.g.,metal,wood) is proportional to the force applied to it. The law laid the basis for studies ofstress and strain and for understanding of elastic materials. He applied these studies in his designs for the balance springs ofwatches; his interest in timekeeping was further reflected in his effort to improve thependulum for clock regulation. In 1662 he was appointedcurator of experiments to theRoyal Society ofLondon and was elected a fellow the following year.

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One of the first men to build aGregorianreflecting telescope, Hooke discovered the fifthstar in the Trapezium, anasterism in theconstellationOrion, in 1664 and first suggested thatJupiter rotates on its axis. His detailed sketches ofMars were used in the 19th century to determine thatplanet’s rate of rotation. In 1665 he was appointed professor ofgeometry in Gresham College. InMicrographia (1665; “Small Drawings”) he included his studies and illustrations of thecrystal structure ofsnowflakes, discussed the possibility of manufacturing artificial fibres by a process similar to the spinning of thesilkworm, and first used the wordcell to name the microscopic honeycomb cavities incork, thereby contributing to the history ofcell theory. His studies of microscopicfossils led him to become one of the firstproponents of a theory ofevolution.

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He suggested that the force ofgravity could be measured by utilizing themotion of a pendulum (1666) and attempted to show thatEarth and theMoon follow an elliptical path around theSun. In 1672 he discovered the phenomenon ofdiffraction (the bending oflight rays around corners); to explain it, he offered thewave theory of light. He stated the inverse square law to describe planetary motions in 1678, a law thatNewton later used in modified form. Hooke complained that he was not givensufficient credit for the law and became involved in bitter controversy with Newton. Hooke was the first man to state in general that allmatter expands when heated and thatair is made up of particles separated from each other by relatively large distances.