1690 – Holland

‘Every point on a wavefront can act as a new source of waves’

A line perpendicular to the wave fronts is called a ray and this ray shows the direction of the wave.

The Huygens construction, published in ‘*Traite de la Lumiere*‘ (’Treatise on Light’, 1690) gives an explanation for the way light is reflected and refracted.

Huygens said that light consists of a disturbance spreading from its source as spherical pressure waves having wave fronts perpendicular to the direction of their motion and correctly anticipated that in a denser medium light would travel more slowly. This hypothesis was largely ignored at the time as it conflicted with NEWTON‘s theory. Huygens’ view, when re-discovered and championed by THOMAS YOUNG (1773-1829) would eventually become the more commonly accepted version.

He invented a pendulum clock (1656) and also discovered Titan, the first observed moon of Saturn (1665).

Huygens discovered that a simple pendulum does not keep perfect time but completes smaller swings faster than big swings. This is because the weight or ‘bob’ of the pendulum follows a circular path. Huygens’ realisation that a pendulum mimicking a circle’s curve does not maintain a perfectly equal swing and that in order to do this it actually needs to follow a ‘cycloidal’ arc, set him on the path to designing the first successful pendulum clock.

Published ‘*Horologium*‘ (1658), ‘*Horologium Oscillatorium*‘ (1673) in which he showed that if the bob’s path were a cycloid (the curved path traced out by a point on the rim of a wheel as it rolls along) instead of a circle, it would be isynchronous (keeping equal time) no matter what the length of the swing. He made the pendulum’s swing cycloidal by suspending a rigid pendulum rod on two chords whose swing either way was limited by two plates called cycloidal checks.

GALILEO had considered the timekeeping possibilities of a swinging pendulum and Huygens successfully tied it with an escapement mechanism.

He explored the mathematics associated with pendulums – which led him, together with HOOKE, to an early prediction of the link between the elliptical orbits of the planets and the inverse square law of gravity. His work was a milestone, playing a key part in the understanding of centrifugal force. It helped to confirm Newton’s laws of motion by showing how an object will travel in a straight line unless pulled into a curved path by some other force.

Huygens was one of the founders of the French Academy of Science.

###### Related sites

- How Stuff Works – Light (science.howstuffworks.com)