Answer
a. To detect movement in doors or windows and trigger an alarm.
b. To detect smoke in the air (interrupting the beam of light) and trigger an alarm.
c. To detect changes in the amount of light hitting the meter.
Work Step by Step
a. Burglar alarm: a beam of light (perhaps not in the visible region) continuously shines on a light-detecting surface. Due to the photoelectric effect, as long as the light shines, electrons are ejected and the resulting small current is amplified.
When a burglar interrupts the beam of light, the small current stops. A logic circuit is triggered and sounds the alarm.
b. Smoke detector: a beam of light continuously shines on a light-detecting surface. Due to the photoelectric effect, as long as the light shines, electrons are ejected and the resulting small current is amplified.
When the air becomes smoky, less light gets through and the small current is diminished. At a certain threshold, a logic circuit is triggered and sounds the alarm.
c. Light meter: the photoelectric current varies as the amount of light falling on the meter changes.
