TeXes Science Practice Test

Which ray strikes a mirror at an angle of 90 degrees?

1. The focal ray

2. The chief ray

3. The parallel ray

4. The emergent ray

The correct answer is: The chief ray

When a ray strikes a mirror at an angle of 90 degrees, it is referred to as the chief ray. This ray is significant in optics because it is typically associated with the center of the aperture of a lens or the principal axis of a mirror. When the chief ray strikes the mirror perpendicularly, it reflects back along the same path, which is a key concept in understanding how mirrors produce images. The other types of rays mentioned do not strike the mirror at a 90-degree angle. For example, the focal ray converges towards the focal point of a mirror but does not necessarily strike at an angle of 90 degrees. The parallel ray, on the other hand, travels parallel to the principal axis before reflecting off the mirror, while the emergent ray is what occurs after light exits a medium and doesn't pertain directly to the incident angles of rays striking a mirror. Thus, recognizing the characteristics of the chief ray and its interaction with the mirror provides a clear understanding of its fundamental role in optics.