Optical or
Photoelectric
Smoke
Detector
Sue Shryock and Sherrie Anderson
Preview
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Inner Works
Lasers
Light Scattering
Demonstration
Helium-Neon Blurb
Sources
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.
Light
Amplification by
Stimulation
Emission of
Radiation
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 HeliumNeon 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 632.8 nm. It can also produce
laser action in the green at 543.5 nm.
The glass tube contains 85% helium
and 15% neon gas at 1/300
atmospheres pressure.
Sources
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Inquiry into Physics 3RD Ed., Ostdiek & Bord
http://education.jlab.org
http://hyperphysics.phy-astr.gsu.edu
http://howstuffworks.com