Photoelectric smoke detectors use a light beam to detect smoke. Specifically, they contain a light source and a light sensor. When smoke enters the detection chamber, it scatters the light beam, causing part of the light to hit the sensor, which triggers the alarm.

Here is a more detailed explanation from anka security:

Light source: Photoelectric detectors contain an LED that emits a light beam.
Light sensor: They are also equipped with a light-sensitive element (photoresistor) that detects the light.
Smoke detection: When smoke enters the detector chamber, it scatters the light beam. Some of this scattered light hits the sensor, which activates the alarm.
Sensitivity to smoldering fires: Photoelectric detectors are generally more sensitive to larger smoke particles produced by smoldering fires, such as a lit cigarette or a couch fire.