A normal section from point A to point B is defined as which of the following?

• A. The intersection of a plane containing the normal at A that passes through B
• B. The line AB
• C. The projection of AB onto the tangent plane at A
• D. A plane perpendicular to AB at its midpoint

Answer: (A)

A normal section is formed by intersecting the surface with a plane that contains the surface normal at the chosen point. For a section from point A toward point B, you pick the plane that includes the normal line at A and also passes through B. The intersection of this plane with the surface gives a curve on the surface—this is the normal section in that direction. This construction is why this option is the correct one: it directly ties the section to the surface normal at A and to the direction toward B.

The other ideas don’t fit the concept. A straight line AB isn’t the surface curve you get from a plane cut. Projecting AB onto the tangent plane at A stays in the tangent plane and doesn’t produce a curve on the surface itself. A plane perpendicular to AB at its midpoint has no relation to the surface normal at A, so it doesn’t define a normal section.