Final answer:
Headlights should be used when visibility is low, typically when you cannot see clearly for 500 feet ahead. The question seems to derive from understanding the physics behind the human eye's resolution limit, which is different from the driving safety regulation.
Step-by-step explanation:
The question is asking at what visibility range headlights should be used. The correct answer is not provided explicitly in the options, as the typical legal requirement is to use headlights when you cannot see clearly for 500 feet ahead of you. However, if we relate this to the physics of the eye's ability to resolve two points of light, such as car headlights, we can determine a related concept. Specifically, we can calculate the greatest possible distance at which a car's headlights, which are 1.3 meters apart, can be resolved by the human eye, considering the diffraction limit of the pupil. This calculation would require the application of the Rayleigh criterion and knowledge of the average wavelength of visible light (approximately 550 nm) as well as the diameter of the pupil (around 0.40 cm).
However, this calculation does not directly answer the question about when to use headlights in terms of visibility distance, which is a driving regulation rather than a physics problem. Still, understanding the optical limits of human vision can be important when considering the safety requirements for headlight usage.