What does a smoke alarm actually detect?
A smoke alarm does not need to recognise a flame. It watches for particles in the air, and two common technologies do that in very different ways.
What is happening?
In a photoelectric alarm, a light beam and a sensor are arranged so that clean air sends little light toward the detector. Smoke particles scatter the light. When enough scattered light reaches the sensor, the electronics trigger the alarm.
Look inside the object
An ionization alarm uses a tiny, shielded radioactive source to ionize air between electrodes, creating a small current. Smoke particles capture ions and reduce that current. The electronics notice the change. Americium-241 has historically been used for this job.
What this object teaches us
The two principles respond differently to particle sizes and fire types, which is why detector design matters. The useful lesson is broader: sensors often do not identify an object directly. They measure a physical change—light, current, resistance, temperature—and infer what is happening.