The normal (ray).
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If the field lines were not perpendicular to the surface, then they could be decomposed into components perpendicular and parallel to the surface. But if there is an E-field along the surface, the surface is no longer an equipotential.
A line is perpendicular to another line when it is at an angle of 90° to the other line. + (that plus sign is an example of a perpendicular line)
A line is perpendicular to a plane when it is perpendicular on two lines from the plane
A perpendicular line is one that is at right angle to another - usually to a horizontal line. A perpendicular bisector is a line which is perpendicular to the line segment joining two identified points and which divides that segment in two.
A sole line can't be perpendicular. To be perpendicular, a line has to meet with another line, to form right angles on either side. Perpendicular is the exact opposite of parallel. Parallel: = Perpendicular: -|