Final answer:
Beam smoke detectors operate on the ionization principle, where ionized air between two metal plates allows a small electric current to flow. When smoke particles enter the detector and interfere with the ions, the current is interrupted, triggering an alarm.
Step-by-step explanation:
The principle on which beam smoke detectors operate is the ionization principle. Inside a smoke detector, there are two metal plates separated by a small distance. A tiny amount of radioactive element, americium, emits alpha particles which ionize the air between the plates. This ionized air allows a small electric current to flow between the plates. When smoke particles enter the detector, they interfere with the ions and interrupt the current flow, triggering an alarm.