Final answer:
The required distance for headlight usage is commonly 500 feet for visibility purposes, but the physical limit of eye resolution depends on the pupil size, the wavelength of light, and the spacing of the headlights.
Step-by-step explanation:
The distance at which headlights must be used varies by jurisdiction, but a common rule is that you must use your headlights between sunset and sunrise, or when visibility is poor. For instance, in many places, headlights are required when you cannot see clearly for at least 500 feet ahead. However, given the references mentioned, this question seems to be integrating concepts of physics, particularly relating to the resolution limit of the human eye based on diffraction through the pupil. The specific distance at which car headlights can be resolved as two separate points of light depends on the diameter of the observer's pupil and the wavelength of light.
Explanation of optical resolution
According to physics, the resolution limit of the human eye can be calculated using the Rayleigh criterion. This criterion states that the minimum angular separation (θ) that the eye can resolve is given by θ = 1.22 λ / D, where λ is the wavelength of light and D is the diameter of the pupil. From previous references, using an average wavelength (λ) of 550 nm and a pupil diameter (D) of 0.40 cm, we can estimate the maximum distance at which car headlights, spaced 1.3 meters apart, can be resolved. While this information provides a scientific basis for understanding visual acuity, it does not directly answer the stated question regarding traffic laws for headlight usage.