Optical or Photoelectric Smoke Detector Sue Shryock and Sherrie Anderson

Preview Inner Works Lasers Light Scattering Demonstration Helium-Neon Blurb Sources

Inner Works Inner Works Often when you walk into a store a bell will go off and you may find a photo beam detector is being used. In this case by walking in you have disrupted the light connection to the photodetector. This is how the smoke detector works.

Continued… Photoelectric smoke detectors have two chambers set at a 90 degree angle to one another. In the top chamber there is a light and in the other is a sensor. In the normal setting, the light shoots left straight across and misses the sensor. When smoke enters the chamber, the smoke particles…

Continued… scatter the light and some of the light then hits the sensor, as shown below. The sensor then sets off the horn. Photoelectric sensors detect slow, smoldering fires. The light source is made from a diode laser..

Why do we use Lasers?

We use Lasers because they can go into a curve and come out in a straight line. They do bend when they hit something but all the light bends the same amount and does not spread out. A property of the electrons within the laser is that after being excited they will return to their original state.

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Light Scattering The Physic behind the Smoke detector

As a light wave travels through the atmosphere, the wave’s oscillating electric field causes the electrons in air molecules to oscillate with the same frequency. These oscillating electrical charges emit electromagnetic radiations (light) that moves in all directions (scattering). Pg. 368

Why is the Sky Blue? Electrons in the air molecules are more efficient at absorbing and radiating higher frequencies of light. When blue light makes an electron oscillate it absorbs and scatters more of the incident radiant energy. The frequency of blue light is about 1.5 times that of red.

Why is the Sunset Red? Affected the ozone and low lying clouds, and with a less amount of blue due to scattering, the red light waves are more abundant and picked up by the air molecules.

Northern Lights This is Earth’s natural version of the way lights scatter when entering our atmosphere.

Demonstration with Helium- Neon Lasers Green Red Yellow

Helium-Neon Blurb The most common and inexpensive gas laser, the helium-neon laser is usually constructed to operate in the red at nm. It can also produce laser action in the green at nm. The glass tube contains 85% helium and 15% neon gas at 1/300 atmospheres pressure.

Sources Inquiry into Physics 3 RD Ed., Ostdiek & Bord