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Brownian Motion

 

If you observe tiny particles, such as pollen or dust, moving in fluids (either liquids and gases) they are seen to move randomly. There is no preferred direction for their motion. This phenomenon is called Brownian motion after the botanist Robert Brown, who first observed this in 1827. He used a microscope to look at pollen grains on the surface of water and observed the motion was totally random in nature. He could not explain why this occurred.

In 1905, physicist Albert Einstein explained the observation by suggesting that the pollen grains were being moved by individual water molecules. This was taken as evidence of the existance of atoms and molecules, providing evidence to support the idea of particle theory.

We now know that pollen grains move on the surface of a dish of water because they are being constantly bombarded by other moving particles in the fluid. These particles are so tiny we cannot seee them. They do not have much mass, but have a lot of momentum because they travel at high speeds. We conclude that the larger particles can be seen moving as they are moved by light, fast-moving molecules.