Which term is a line perpendicular to a surface?

Which term is a line perpendicular to a surface?

• A. Focal point
• B. Angle of reflection
• C. Index of refraction
• D. Normal line

Answer: (D)

A line perpendicular to a surface is called the normal line. It serves as the reference for measuring how light interacts with the surface, because angles of incidence and reflection (and refraction) are always defined with respect to this line. The focal point is a spot where light converges, not a line; the angle of reflection is an angle formed between the reflected ray and the normal, not a line; and the index of refraction is a property of a material that describes how fast light travels in it, not a line. So the term that describes a line perpendicular to the surface is the normal line.

A line perpendicular to a surface is called the normal line. It serves as the reference for measuring how light interacts with the surface, because angles of incidence and reflection (and refraction) are always defined with respect to this line. The focal point is a spot where light converges, not a line; the angle of reflection is an angle formed between the reflected ray and the normal, not a line; and the index of refraction is a property of a material that describes how fast light travels in it, not a line. So the term that describes a line perpendicular to the surface is the normal line.