What triggers the alarm in a beam detector?

A beam detector operates based on the principle of light transmission. In this system, a light beam is projected between a transmitter and a receiver. The alarm is triggered when a solid object, such as a person or an obstruction, blocks this light path. This interruption in the light beam indicates a potential hazard in the area being monitored, leading to the activation of the alarm.

While smoke detectors detect airborne particles, and heat detectors respond to changes in temperature, the function of a beam detector specifically relies on the light being obstructed. Consequently, options related to smoke, heat, and steam do not pertain to the core operation of a beam detector, as these factors do not influence the light beam directly. Thus, the correct answer highlights the beam detector's reliance on the presence of a solid object to trigger its alarm.