Caption: Portrait of Christiaan Huygens (1629--1695).

People in those days went in for lace. I think the hair is fake.

Huygens, among many other great works, discovered the eponymous Huygens principle, was one of early pioneers of the wave theory of light.

He was preceded by Francesco Maria Grimaldi (1618--1663)---who discovered the diffraction of light---and Ignace-Gaston Pardies (1636--1673)---who seems to be the originator of the wave theory of light.

Credit/Permission: Bernard Vaillant (1632--1698, 17th century (uploaded to Wikimedia Commons by User:Lord Horatio Nelson, 2008) / Public domain.
Image link: Wikimedia Commons: File:Christiaan Huygens-painting.jpeg.
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