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The dot product between two vectors is only equivalent to the cosine of the
angle if the two vectors are unit vectors. So the two vectors
should be interpolated in a way that the interpolant
is moving uniformly between the two vectors and its length
remains one. As  point out this interpolations works on the
surface of the unit sphere and therefore it is called spherical
interpolation (Formula 3).
. Unfortunately spherical
interpolation is not available on current graphics accelerators, only
linear interpolation is supported. When using linear interpolation this
results in denormalized vectors (Figure 4) and
wrong shading intensities.
Linear Interpolation versus Spherical Interpolation