Law of Reflection
The law of reflection states that the angle of reflection equals the angle of incidence: θr = θi. The angles are measured relative to the perpendicular to the surface at the point where the ray strikes the surface.
Source
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"OpenStax College, College Physics. November 10, 2012."
http://cnx.org/content/m42456/latest/?collection=col11406/1.7
OpenStax CNX
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