Final answer:
Parallel light rays reflecting from a plane mirror remain parallel because the surface is smooth, respecting the law of reflection, where the angle of incidence equals the angle of reflection. Only observers at the certain reflective angle can see the light.
Step-by-step explanation:
When parallel light rays are incident on the surface of a plane mirror, the rays will reflect in such a way that the angle of reflection equals the angle of incidence. This means that parallel rays will remain parallel after reflecting off a plane mirror. The reason for this is that a plane mirror has a very smooth surface which allows the light to reflect in only one direction as dictated by the law of reflection. Thus, an observer would need to be at the specific reflective angle to see the light. In contrast, a rough surface like paper would diffuse the light, allowing it to be seen from many different angles.
For example, Figure 1.9 illustrates a ray of light that strikes two mutually perpendicular reflecting surfaces and is reflected back exactly parallel to its original direction, which is how corner reflectors work on bicycles and cars.