There would have to be a LOT of smoke before the alarm would go off. Photoelectric smoke detectors therefore use light in a different way. Inside the smoke detector there is a light and a sensor, but they are at 90 degree angles to one another. In the normal case, the light from the light source on the left shoots straight across and misses the sensor. When smoke enters the chamber, however, the smoke particles scatter the light and some amount of light hits the sensor and triggers the alarm.
Photoelectric detectors are better at sensing smoky fires, such as a smoldering mattress.
I n the late 1930s the Swiss physicist Walter Jaeger tried to invent a sensor for poison gas. He expected that gas entering the sensor would bind to ionized air molecules and thereby alter an electric current in a circuit in the instrument. Smoke particles had apparently done what poison gas could not. Jaeger's experiment was one of the advances that paved the way for the modern smoke detector. It was 30 years, however, before progress in nuclear chemistry and solid-state electronics made a cheap sensor possible.