Final answer:
When looking at a folded index card through a pinhole, the border appears as a reflectance edge because the card appears flat. This is explained by the principles of geometric optics and the law of reflection.
Step-by-step explanation:
The best option for the given question, 'If you look at a folded index card though a pinhole, you see the border as a(n) _____ because the card looks _____.' would be c. reflectance edge; flat.
The principle in operation here is geometric optics, a field of physics that describes light propagation in terms of rays. The border of the card would present as a 'reflectance edge'. The term 'reflectance edge' pertains to the boundary area where light is reflected off a surface. When viewing a folded index card through a pinhole, the card appears flat instead of 3-D and its edges seem to form clear lines as the light reflects off the surface of the card.
These mirror-like reflections are due to the flat, smooth surface of the index card, as per the principles of reflection of light. A crucial aspect of this phenomenon is the law of reflection which states that the angle of reflection equals the angle of incidence.
Learn more about light reflection