Huygens's Refraction
Huygens's principle applied to a straight wavefront traveling from one medium to another where its speed is less. The ray bends toward the perpendicular, since the wavelets have a lower speed in the second medium.
Source
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"OpenStax College, Huygens's Principle: Diffraction. January 11, 2013."
http://cnx.org/content/m42505/latest/
OpenStax CNX
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