### Topic Page: Huygens, Christiaan, 1629-1695

**Huygens, Christiaan 1629-1695,**from

*Dictionary of Energy*

**Huygens, Christiaan (1629-1695)**

*The Hutchinson Dictionary of Scientific Biography*

Place: Netherlands

Subject: biography, physics

Dutch physicist and astronomer. He is best known in physics for his explanation of the pendulum and invention of the pendulum clock, and for the first exposition of a wave theory of light. In astronomy, Huygens was the first to recognize the rings of Saturn and discovered its satellite Titan.

Huygens was born on 14 April 1629 at The Hague into a prominent Dutch family with a tradition of diplomatic service to the ruling house of Orange, and a strong inclination towards education and culture. René Descartes was a frequent guest at the house of Huygens's father, Constantijn, who was a versatile and multi-talented man, a diplomat, a prominent Dutch and Latin poet, and a composer. So it was natural for the young Christiaan to be educated at home to the highest standard. In 1645 he was sent to the University of Leiden to study mathematics and law, followed in 1647 by two years studying law at Breda.

But, eschewing his expected career in diplomacy, Huygens returned to his home to live for 16 years on an allowance from his father that enabled him to devote himself to his chosen task, the scientific study of nature. This long period of near seclusion was to be the most fruitful period of his career.

In 1666 on the foundation of the Académie Royale des Sciences, Huygens was invited to Paris and lived and worked at the Bibliothèque Royale for 15 years, until his delicate health and the changing political climate took him back to his home. Here he continued to experiment, only occasionally venturing abroad to meet the other great scientists of the time as in 1689, when he visited London and met Isaac Newton. During his stay in Paris, Huygens twice had to return home for several months because of his health and in 1694 he again fell ill. This time he did not recover, and he died in The Hague on 8 July 1695.

Huygens worked in different areas of science in a way almost impossible in our modern age of vast knowledge and increasing specialization. He made important advances in pure mathematics, applied mathematics, and mechanics, (which he virtually founded); optics and astronomy, both practical and theoretical; and mechanistic philosophy. He is also credited with the invention of the pendulum clock, enormously important for its use in navigation, since accurate timekeeping is necessary to find longitude at sea. In a seafaring nation such as Holland, this was of particular importance. Typically of Huygens, he developed this work into a thorough study of pendulum systems and harmonic oscillation in general.

At first Huygens concentrated on mathematics. In the age of such revolutionary figures as Descartes, Newton and Gottfried Leibniz, his career may be termed conservative for it contained nothing completely new except the theory of evolutes and Huygens's study of probability including game theory, which originated our modern concept of the expectation of a variable. His suspicion of the new methods may have been partly due to the secrecy of scientists of his time, and partly to Huygens's fastidiousness, which often led him to delay publishing his observations and theories. The importance of his mathematical work is in its improving on available techniques, and Huygens's application of them to find solutions to problems in science.

The first of his many studies in applied mathematics was a paper on hydrostatics, including much mathematical analysis, published in 1650. Fascinating work on impact and collision followed, motivated by Huygens' disbelief in Descartes's laws of impact. Huygens used the idea of relative frames of reference, considering the motion of one body relative to the other. He discovered the law of conservation of momentum, but as yet the vectorial quantity had little intuitive meaning for him and he did not proceed beyond stating the law as the conservation of the centre of gravity of a system of bodies under impact. In De motu corporum (1656) he was also able to show that the quantity 1/2mv^{2} is conserved in an elastic collision, though again this concept had little intuitive sense for him.

Huygens also studied centrifugal force and showed, in 1659, its similarity to gravitational force, though he lacked the Newtonian concept of acceleration. Both in the early and the later part of his career he considered projectiles and gravity, developing the mathematically primitive ideas of Galileo, and finding in 1659 a remarkably accurate experimental value for the distance covered by a falling body in one second. In fact his gravitational theories successfully deal with several difficult points that Newton carefully avoided. Later, in the 1670s, Huygens studied motion in resisting media, becoming convinced by experiment that the resistance in such media as air is proportional to the square of the velocity. Without calculus, however, he could not find the velocity-time curve.

Early in 1657, Huygens developed a clock regulated by a pendulum. The idea was patented and published in the same year, and was a great success; by 1658 major towns in Holland had pendulum tower clocks. Huygens worked at the theory first of the simple pendulum and then of harmonically oscillating systems throughout the rest of his life, publishing the Horologium oscillatorium in 1673. He made many technical and theoretical advances, including the derivation of the relationship of the period T of a simple pendulum to its length l as:

T = 2π√(l/g)

Huygens made use of his previously worked out theory of evolutes in this work.

Huygens is perhaps best known for his wave theory of light. This was the result of much optical work, begun in 1655 when Huygens and his brother began to make telescopes of high technical quality.

Through working with his brother, Constantijn, Huygens became skilful in grinding and polishing lenses, and the telescopes that the two brothers constructed were the best of their time. Huygens's comprehensive study of geometric optics led to the invention of a telescope eyepiece that reduced chromatic aberration. It consisted of two thin plano-convex lenses, rather than one fat lens, with the field lens having a focal length three times greater than that of the eyepiece lens. Its main disadvantage was that cross-wires could not be fitted to measure the size of an image. To overcome this problem Huygens developed a micrometer, which he used to measure the angular diameter of celestial objects.

Having built (with his brother) his first telescope, which had a focal length of 3.5 m/11.5 ft, Huygens discovered Titan, one of Saturn's moons, in 1655. Later that year he observed that its period of revolution was about 16 days and that it moved in the same plane as the so-called ‘arms’ of Saturn. This phenomenon had been somewhat of an enigma to many earlier astronomers, but because of Huygens's superior telescope, and a piece of sound if not brilliant deduction, he partially unravelled the detail of Saturn's rings. In 1659 he published a Latin anagram that, when interpreted, read ‘It (Saturn) is surrounded by a thin flat ring, nowhere touching and inclined to the ecliptic’. The theory behind Huygens's hypothesis followed later in Systema Saturnium, which included observations on the planets, their satellites, the Orion nebula and the determination of the period of Mars. The content of this work amounted to an impressive defence of the Copernican view of the Solar System.

The Traité de la lumière, containing Huygen's famous wave, or pulse, theory of light, was published in 1690. Huygens had been able two years earlier to use his principle of secondary wave fronts to explain reflection and refraction, showing that refraction is related to differing velocities of light in media, and his publication was partly a counter to Newton's particle theory of light. The essence of his theory was that light is transmitted as a pulse with a ‘tendency to move’ through the ether by setting up a whole train of vibrations in the ether in a sort of serial displacement. The thoroughness of Huygens's analysis of this model is impressive, but although he observed the effects due to polarization, he could not yet use his ideas to explain this phenomenon.

The impact of Huygens's work in his own time, and in the 18th century, was much less than his genius deserved. Essentially a solitary man, he did not attract students or disciples and he was also slow to publish his findings. Nevertheless, after Galileo and until Newton, he was supreme in mechanics, and his other scientific work has had a significant effect on the development of physics.

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