Huygens, Christiaan

Dutch mathematical physicist and astronomer. He proposed the wave theory of light, developed the pendulum clock in 1657, discovered polarization, and observed Saturn's rings. He made important advances in pure mathematics, applied mathematics, and mechanics, which he virtually founded. His work in astronomy was an impressive defence of the Copernican view of the Solar System.

Mechanics
Huygens' first studies in applied mathematics dealt with mechanics, the branch of physics pertaining to motions and forces. Working on impact and collision, Huygens used the idea of relative frames of reference, considering the motion of one body relative to the other. He anticipated the law of conservation of momentum stating that in a system of bodies under impact the centre of gravity is conserved. In De Motu Corporum 1656, he was also able to show that the quantity 1/2mv² is conserved in an elastic collision.

Huygens also studied centrifugal force and showed, in 1659, its similarity to gravitational force, although he lacked the Newtonian concept of acceleration. He considered projectiles and gravity, developing the mathematically primitive ideas of Galileo. He found an 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. 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.

The pendulum clock
In 1657, Huygens developed a clock regulated by a pendulum, an idea that he published and patented. 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 derived the relationship between the period of a simple pendulum and its length.

The theory of light
The Traité de la Lumière/Treatise on Light (1690) contained Huygens' famous wave or pulse theory of light. Two years earlier, Huygens had been able 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. He theorized that light is transmitted as a pulse moving through a medium, or ether, by setting up a whole train of vibrations in the ether in a serial displacement. His publication was partly a counter to Newton's particle theory of light. The thoroughness of Huygens' 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.

Astronomy and the telescope
Huygens' 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. Huygens then developed a micrometer to measure the angular diameter of celestial objects.

With a home-made telescope, he discovered Titan, one of Saturn's moons, in 1655. Later that year he observed that Titan's 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' superior 7-m telescope, 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' hypothesis followed later in Systema Saturnium (1659), which included observations on the planets, their satellites, the Orion nebula and the determination of the period of Mars, and provided further evidence for the Copernican view of the Solar System.