Which principle is used by a photoelectric smoke detector to detect smoke?

• A. The detector uses temperature rise to trigger a thermal switch.
• B. Smoke refracts light to trigger a photocell.
• C. The detector uses gas detection for smoke.
• D. The detector uses ultrasonic waves.

Answer: (B)

Photoelectric smoke detectors detect smoke by how smoke affects the light path inside the sensing chamber. A light beam and a photocell are arranged so that, in clean air, the light doesn’t reach the sensor. When smoke enters, the particles scatter light and redirect some of it toward the photocell. The photocell detects this change and triggers the alarm. This is the principle described by the statement that smoke interacts with light to produce a signal for the sensor. The other techniques mentioned—thermal switches triggered by heat, gas sensing, or ultrasonic waves—belong to different types of detectors and are not how a photoelectric smoke detector works.