Physics Photo of the Week
February
29, 2008
Smoke Particles and Brownian Motion -
Discussion by Robin Dhakal
Brownian
motion (or Brownian movement) can be defined as "the random movement of
microscopic particles suspended in a fluid." It is the result of
asymmetry in the kinetic impacts of molecules that make up the liquid.
In 1847, biologist Robert Brown noticed that the pollen grains in water
jiggle. But, he was unable to give the explanation for that. Einstein,
in 1905 gave the explanation on the basis of molecular theory. He
explained that the jiggling of the pollen grains seen in Brownian
motion was due to molecules of water hitting the tiny pollen grains.
In this experiment the smoke from a match is trapped with air in a cell
fitted with windows so that the smoke particles can be microscopically
observed. To see a clear picture of the smoke particles, a beam of
laser light is passed through it. The red smoke particles are seen in
the picture and the video clip at right 
because
of the laser light. When viewed in the microscope, the smoke particles
were seen to be floating and moving in a random pattern as seen in the
video. This can be explained on the basis of Einstein’s
explanation of Brownian Motion. This random movement pattern of the
smoke particles is due to the molecules of the air hitting the smoke
particles constantly. In this case, the air molecules are causing the
smoke particles to move randomly. Hence, they are in constant motion in
a random pattern.
Some of the smoke particles are in clear focus as point-like
objects. Most particles, however are either too close or too far
from the microscope to be in clear focus.
Physics
Photo of the
Week is
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
Department. These photos feature an interesting phenomena in
the world around us. Students, faculty, and others are invited to
submit digital (or film) photographs for publication and
explanation. Atmospheric phenomena are especially welcome.
Please send any photos to dcollins@warren-wilson.edu.
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here to see the Physics Photo
of
the Week Archive.
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