A line perpendicular to the surface at the point of incidence.
Explanation
In geometric optics, the behavior of light upon striking a surface is governed by the laws of reflection. To accurately describe the path of light and measure angles, specific geometric reference lines are established relative to the surface.
Definition of Normal:
The Normal is an imaginary line drawn perpendicular (at a 90-degree angle) to the reflecting surface at the exact point where the light ray strikes, known as the point of incidence. It serves as the standard reference for measuring optical angles.
Analysis of Options:
- The path of the incident light ray. is incorrect: The path of the incident light ray is simply called the incident ray.
- A line parallel to the reflecting surface. is incorrect: A line parallel to the surface is a tangent, not the normal.
- A line perpendicular to the surface at the point of incidence. is correct: The normal is defined strictly as the line perpendicular to the surface at the point of incidence.
- The path taken by the reflected ray. is incorrect: The path taken by the light after striking the surface is called the reflected ray.
Key Takeaway:
The Normal is the perpendicular reference line at the point of incidence; the angle of incidence and the angle of reflection are always measured with respect to the normal, not the surface itself.