Which type of smoke detector operates when either light smoke or heavy, dense smoke blocks a part of the photoelectric beam?

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Understanding how photoelectric smoke detectors respond helps explain this choice. There are two main styles: light obscuration and light scattering. A light obscuration detector works by having a beam of light travel to a sensor; when smoke enters, it blocks part of that beam. If enough of the beam is blocked—whether the smoke is light or dense—the sensor triggers the alarm. That’s exactly what the scenario describes: the detector operates when smoke blocks a portion of the photoelectric beam, regardless of smoke density.

In contrast, a light scattering detector triggers when smoke particles scatter light from the emitter toward the sensor, not merely by blocking the beam. The other options refer to different technologies: infrared flame detectors look for flame radiation, and a liquid expansion fixed‑temperature device is a type of thermal detector, not a photoelectric smoke detector.