Brownian movement

The irregular movement of dust particles becomes visible in the air when the particles are caught in a ray of sunlight. The tiny dust particles move randomly as they are buffeted by gas molecules in the atmosphere, which are too small to be seen with the naked eye.

Continuous random motion of particles in a fluid medium (gas or liquid) as they are subjected to impact from the molecules of the medium. The phenomenon was explained by German physicist Albert Einstein in 1905 but was observed as long ago as 1827 by the Scottish botanist Robert Brown. Brown was looking at pollen grains in water under a microscope when he noticed the pollen grains were in constant, haphazard motion. The motion of these particles was due to the impact of moving water molecules. It provides evidence for the kinetic theory of matter.

In order for the irregular motion to be observed, the particles in the medium must be sufficiently small relative to the bombarding molecules for the impact of the bombarding molecules to have an effect. A tennis ball in air, for instance, would not show Brownian motion because the impacts of the moving air molecules on one side of the tennis ball would be balanced by impacts of the molecules on the other side. In other words, the resultant force of the impacts would be too small.