= Christiaan Huygens
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<Christiaan Huygens> (1629–1695) connected mathematical <mechanics> with the design of a more accurate <pendulum clock>, and worked on <astronomy>, <probability> and the wave account of <light>. His optical account treats a <wavefront> as generating secondary wavelets, the geometric idea behind <Huygens' principle>.https://mathshistory.st-andrews.ac.uk/Biographies/Huygens/[University of St Andrews, MacTutor biography, pendulum-clock work and discussion of the wave theory of light]{ref}
The clock provides a useful scientific motivation: a small-amplitude <simple pendulum> has period approximately $2\pi\sqrt{\ell/g}$, so a stable length supplies a repeatable time scale. Finite-amplitude corrections show why an approximate classroom formula does not by itself produce a perfect timekeeper. In <wave propagation>, the wavefront construction addresses a different problem: how a disturbance advances through space and changes direction under <refraction>.
\Image[https://commons.wikimedia.org/wiki/Special:Redirect/file/Museum%20Boerhaave%20-%20Portrait%20Christiaan%20Huygens.jpg?width=250]
{title=Portrait of <Christiaan Huygens>}
{description=Photograph by Prof. Jos van den Broek; Museum Boerhaave, Leiden. https://creativecommons.org/licenses/by-sa/3.0/[CC BY-SA 3.0], via Wikimedia Commons.}
{source=https://commons.wikimedia.org/wiki/File:Museum_Boerhaave_-_Portrait_Christiaan_Huygens.jpg}
{width=250}
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