The angle of incidence is measured with respect to what line?

• A. The boundary surface
• B. The boundary's normal
• C. The transmitted ray
• D. The reflected ray

Answer: (B)

The angle of incidence is the angle between the incoming ray and the boundary’s normal—the line perpendicular to the surface at the point where the ray meets the boundary. This normal provides a fixed reference that stays the same no matter how the surface is oriented, giving a consistent way to define angles at the point of contact. That consistency is why reflection and refraction use angles measured from the normal: for reflection, the incident angle equals the reflected angle; for refraction, Snell’s law relates the sine of the angle in each medium, both measured from the normal. If you tried to measure from the surface itself, the reference would change with surface orientation (especially on curved boundaries), and the fundamental relationships would no longer hold.