Normal (geometry) Normal (geometry) In geometry, a normal is an object such as a line or vector that is perpendicular to a given object. For example, in two dimensions, the normal line to a curve at a given point is the line perpendicular to the tangent line to the curve at the point. In three dimensions, a surface normal, or simply normal, to a surface at a point "P" is a vector that is perpendicular to the tangent plane to that surface at "P". The word "normal" is also used as an adjective: a line normal to a plane, the normal component
Line (geometry) ray, the "AD" ray is called the "opposite ray". Thus, we would say that two different points, "A" and "B", define a line and a decomposition of this line into the disjoint union of an open segment and two rays, "BC" and "AD" (the point "D" is not drawn in the diagram, but is to the left of "A" on the line "AB"). These are not opposite rays since they have different initial points. In Euclidean geometry two rays with a common endpoint form an angle. The definition of a ray depends upon the notion of betweenness for points on