Concept
Version 8
Created by Boundless
Tangent Vectors and Normal Vectors
Figure 2: Normal Plane
A plane can be determined as normal to the object if the directional vector of the plane makes a right angle with the object at its tangent point. This plane is normal to the point on the sphere to which it is tangent. Each point on the sphere will have a unique normal plane.
Source
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"Surface normal illustration."
http://en.wikipedia.org/wiki/File:Surface_normal_illustration.png
Wikipedia
Public domain.