What do passive infrared sensors detect in security systems?

Passive infrared sensors are designed to detect motion by monitoring changes in infrared radiation. These sensors are focused on the heat emitted by objects, such as humans or animals, in their field of view. When a warm body moves through a cooler area, the sensor picks up these fluctuations in infrared radiation and triggers an alert, indicating motion has been detected. This functionality is critical in security systems as it allows for effective monitoring of areas without requiring direct light or sound detection.

The other options do not align with the operational principles of passive infrared sensors. They either focus on detecting different forms of energy or phenomena that passive infrared sensors are not designed to recognize, which reinforces why option B is the appropriate choice.