Final answer:
Red back reflector light must be visible for a certain distance in order to ensure safety on the road.
Step-by-step explanation:
The red back reflector light must be visible for a certain distance in order to ensure safety on the road. The specific distance for visibility depends on various factors such as the intensity of the light, the size and position of the reflector, and the ambient lighting conditions. For example, in the United States, the Federal Motor Vehicle Safety Standards (FMVSS) dictate that red reflector lights on the rear of a vehicle must be visible from a minimum distance of 1000 feet.