State the law of reflection and give an example of how it applies to a plane mirror.

• A. The angle of incidence equals the angle of reflection; the incoming ray and the reflected ray make equal angles with the normal
• B. The angle of incidence is always greater than the angle of reflection
• C. The angle of incidence equals the angle of reflection; the incoming ray and the reflected ray lie on the same side of the normal
• D. The angle of incidence equals the angle of reflection; the reflected ray is perpendicular to the normal

Answer: (A)

Light follows a simple rule when it hits a smooth surface: the angle of incidence equals the angle of reflection, and both angles are measured with respect to the normal (a line perpendicular to the surface at the point of contact). In a plane mirror, the incident ray approaches from one side of the normal and the reflected ray leaves on the opposite side, and the two angles to the normal are the same. For example, if the incoming ray makes 25 degrees with the normal, it will reflect at 25 degrees on the other side of the normal. This keeps the path symmetric about the normal, and both rays lie in the same plane defined by the surface and the normal. Other statements conflict with this symmetry: the incidence angle isn’t greater than the reflection angle, the rays don’t lie on the same side of the normal, and the reflected ray isn’t necessarily perpendicular to the normal.