What is the imaginary line perpendicular to a boundary surface called?

• A. Tangent
• B. Normal
• C. Radius
• D. Axis

Answer: (B)

The line perpendicular to a boundary surface at a point is called the normal. It forms a right angle with the surface, serving as the reference direction for how light meets or leaves that surface—important in optics and facet alignment. A tangent, by contrast, runs along the surface within its plane. A radius connects the center of a circle or sphere to its boundary, not the orientation of a surface at a boundary point. An axis is a line about which an object rotates or has symmetry. So the imaginary line perpendicular to the surface is the normal.