Visible products of combustion are best detected by a photoelectric detector.

• A. Ionization Detector
• B. Heat Detector
• C. Smoke Detector
• D. Photoelectric Detector

Answer: (D)

The key idea is how smoke particles signal a fire. Photoelectric detectors use a light source and a sensor; when visible smoke enters the chamber, the smoke particles scatter the light, and some of that scattered light reaches the sensor, triggering the alarm. This makes them especially responsive to smoky, smoldering fires where visible smoke is abundant.

Ionization detectors, by contrast, rely on ionizing air to detect changes in current and are more responsive to fast, flaming fires that produce tiny, invisible particles. Heat detectors react to high temperatures and don’t detect smoke at all. Since the question concerns visible products of combustion, the light-scattering mechanism of a photoelectric detector makes it the best choice.