Danish philosopher who discovered double refraction in a crystal of Iceland spar (calcite, CaCO₃). He termed the two rays the ordinary and extraordinary ray. He was not able to explain the phenomenon, which results from the two polarizations of light having different refractivities, and it remained a mystery until Young's wave theory of light. Huygens also investigated the phenomenon.

Huygens, Young

Additional biographies: MacTutor (St. Andrews)