Angle of incidence is defined as the angle at which a ray of light strikes a surface measured from the normal.

Angle of incidence is defined as the angle at which a ray of light strikes a surface measured from the normal.

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

Answer: (A)

The angle of incidence is defined by the direction of the incoming ray relative to the line perpendicular to the surface, known as the normal. This reference is used because the normal provides a fixed, well-defined direction at the point where the ray hits, which keeps the measurement consistent regardless of the surface’s tilt. Using the normal aligns with the fundamental laws of optics, such as the law of reflection and Snell’s law for refraction, which rely on angles measured from this perpendicular direction. If you tried measuring from the surface itself, you’d be looking at the complement (90° minus the incidence angle), which would complicate comparisons between incident and reflected or refracted rays. The tangent line (along the surface) would similarly give a different, less natural reference, and an axis has no relation to the surface geometry in this context.

The angle of incidence is defined by the direction of the incoming ray relative to the line perpendicular to the surface, known as the normal. This reference is used because the normal provides a fixed, well-defined direction at the point where the ray hits, which keeps the measurement consistent regardless of the surface’s tilt. Using the normal aligns with the fundamental laws of optics, such as the law of reflection and Snell’s law for refraction, which rely on angles measured from this perpendicular direction.

If you tried measuring from the surface itself, you’d be looking at the complement (90° minus the incidence angle), which would complicate comparisons between incident and reflected or refracted rays. The tangent line (along the surface) would similarly give a different, less natural reference, and an axis has no relation to the surface geometry in this context.