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

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