Final answer:
Headlights must generally be used from one half-hour after sunset to one half-hour before sunrise. This is based on natural light conditions and legal requirements for safe driving during low-light times.
Step-by-step explanation:
The times during which headlights must be used can vary based on jurisdiction, but generally, they should be active during low-light conditions for safety. The best answer to the question about when headlights must be used during which times is typically one half-hour after sunset to one half-hour before sunrise. This time frame is dictated by the fact that it is after dawn and before dusk that natural lighting conditions are diminished enough to require artificial illumination for safe driving. Additionally, it is also common for laws to require the use of headlights during any conditions of reduced visibility, such as rain, fog, or snow, no matter the time of day.
It's worth noting that the atmosphere does indeed provide some twilight illumination when the Sun is below the horizon by scattering sunlight. This can extend the period of natural light before sunrise and after sunset—times when headlights may not be strictly necessary, but their usage improves visibility for the sake of safety.