Editing Huygens–Fresnel principle (section)

===Refraction===
The apparent change in direction of a light ray as it enters a sheet of glass at angle can be understood by the Huygens construction. Each point on the surface of the glass gives a secondary wavelet. These wavelets propagate at a slower velocity in the glass, making less forward progress than their counterparts in air. When the wavelets are summed, the resulting wavefront propagates at an angle to the direction of the wavefront in air.<ref name=Wood-1905> 
{{cite book|author=Wood, R. W.|title=Physical Optics|year=1905|isbn=978-1-172-70918-2|publisher=MacMillan|location=New York|url=https://archive.org/details/physicaloptics00wooduoft}}</ref>{{rp|56}}

[[File:Refraction - Huygens-Fresnel principle.svg|thumb|Wave [[refraction]] in the manner of Huygens|center]]
In an inhomogeneous medium with a variable index of refraction, different parts of the wavefront propagate at different speeds. Consequently the wavefront bends around in the direction of higher index.<ref name=Wood-1905/>{{rp|68}} 
[[File:HuygensRefractionVariableIndex.svg|thumb|Huygens-Fresnel construction of wave refraction in a medium with variable index of refraction|center]]
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