What do photoelectric smoke detectors and alarms rely on for operation?

Photoelectric smoke detectors operate using a beam of light to detect smoke particles in the air. This type of detector has a light source and a photosensitive sensor. Under normal conditions, light from the source does not reach the sensor directly, as it's positioned at an angle away from the beam. However, when smoke enters the chamber, it scatters the light, directing some of it towards the sensor. When this scattering effect occurs, the sensor detects the change in light and triggers an alarm.

This operational principle is particularly effective for detecting smoldering fires that produce larger smoke particles, as these are more likely to scatter light. In contrast, the other options are based on different principles and mechanisms that do not pertain to the specific functionality of photoelectric detectors. Heat sensors detect temperature changes, sound waves are used in ultrasonic detection devices, and chemical reactions play a role in certain types of smoke and gas detecting systems, but none of these align with the light-based operation of photoelectric smoke detectors.