Huygens , Christiaan

(1629–1695) Dutch physicist and astronomer

Huygens, whose father was the famous Renaissance poet Constantin Huygens, was born in The Hague and studied at the University of Leiden and the College of Breda. He worked in Paris as one of the founding members of the French Academy of Sciences from 1666 to 1681 when, as a Protestant, he found the growing religious intolerance threatening, and returned to The Hague. His first work was in mathematics, but his greatest achievements were in physical optics and dynamics, and his importance to 17th-century science is second only to that of Newton.

Huygens's first great success was the invention of the pendulum clock. Galileo had noted in 1581 that a pendulum would keep the same time whatever its amplitude. Many, including Galileo himself, had tried unsuccessfully to use this insight to construct a more reliable clock. Huygens showed that a pendulum that moves in the arc of a circle does not move with an exactly equal swing. To produce an isochronous (equal-timed) swing it would need to move in a curve called a cycloid. It should be emphasized that Huygens worked this out largely from first principles. He also showed how the pendulum could be constructed so to move in a cycloidal path and how to make the connection to the escapement. The first clock was made to his design by Salomon Coster in 1657 and was described in Huygens's book Horologium (1658; The Clock). The pendulum became one of the basic tools of 17th-century scientific investigation.

Huygens also made major contributions to astronomy as a designer of improved telescopes and as an observer of Saturn. He discovered Titan, Saturn's largest satellite, in 1655 and after prolonged observation was able to describe Saturn's rings correctly.

In 1673 Huygens published Horologium oscillatorium (The Clock Pendulum), a brilliant mathematical analysis of dynamics, including discussions of the relationship between the length of a pendulum and its period of oscillation, and the laws of centrifugal force. It also included an early formulation of Newton's first law of motion: that without some external force, such as gravity, a body once set in motion would continue in a straight line. His views on gravity were worked out in Discours de la cause de la pesanteur (1690; Discourse on the Cause of Gravity). As a Cartesian (a follower of René Descartes) he could not accept Newtonian action at a distance or, in fact, any talk of forces. Instead he would only accept a mechanical explanation, which meant a return to some kind of vortex theory. That is, bodies can only be heavy not because they are attracted by another body but because they are pushed by other bodies.

Huygens's greatest achievement was his development of the wave theory of light, described fully in his Traité de la lumière (1690; Treatise on Light). He assumed that space was pervaded by ether formed of particles, the disturbance of which constituted the radiation of light with the disturbance of one particle being passed on to its neighbor and so on. The disturbances can be considered as waves spreading in a regular spherical form from the point of origin – the particles disturbed in phase constituting a wave front. Each point on a wave front may be regarded as a source of new secondary wavelets and a surface tangent joining such wavelets (i.e., the envelope of the secondary wavelets) can be considered as a new wave front. This method of treating light waves is known as the Huygens construction. Using it, Huygens dealt with reflection and refraction and predicted – as Newtonian theory did not – that light should travel more slowly in a denser medium. But as Huygens considered the waves to be longitudinal, the theory could not explain polarization.

Newton's Opticks (1704) presented a corpuscular (particle) theory of light, and the wave theory lay dormant until it was taken up by Thomas Young and his contemporaries.