0 degrees
Explanation
The behavior of light striking a reflective surface is governed by the Laws of Reflection. The primary law relevant here states that the angle of incidence is always equal to the angle of reflection.
Detailed Analysis:
- Definition of Angles: In optics, angles are measured relative to the normal (an imaginary line perpendicular to the surface at the point of incidence), not relative to the surface itself.
- Angle of Incidence ($i$): Since the ray strikes the mirror along the normal, the angle between the incident ray and the normal is $0^\circ$.
- Application of Law: According to the law of reflection, the angle of incidence ($i$) equals the angle of reflection ($r$). Therefore, since $i = 0^\circ$, it follows that $r = 0^\circ$.
- Result: The light ray reflects back along the same path (retraces its path).
A common error is to confuse the angle of the ray with the mirror surface ($90^\circ$) with the angle of incidence. While the ray is perpendicular to the surface, the angle of incidence is defined as $0^\circ$.
Key Takeaway:
When a light ray is incident normally on a plane mirror, the angle of incidence is $0^\circ$, resulting in an angle of reflection of $0^\circ$.