In 1827, the Scottish Botanist Robert Brown discovered that fine pollen grains suspended in water was always moving in a zigzag and haphazard manner covering very short distances and where never stopping. This random and haphazard movement came to be called Brownian motion.

An Experiment to Observe Brownian Motion

• Requirement: microscope, lens, glass cell, smoke, light souce.
• Diagram:

• Procedure:

Now adjust the microscope slightly util you can see very bright graphite or smoke particles. The particles of graphite (or smoke ) scatter (reflect) the light shining on them and soappears as bright points of light darting about in a random or erratic motion. Note that the graphite ( or smoke) particles are much larger than the water (or air) molecules. The particles can be seen by the light they scatter but the molecules themselves are too small to be seen.

• Explanation:

The irregular movement of the visible particles of graphite axplained as being due to an uneven bombardment of the particles by the invisible molecules of water or air. The irregularpath followed by an individual smoje or graphite particle is shown below.

Factors Affecting Brownian Motion

• The size of the Particles: Isf the particle is small, Brownian motion will be observed and if the particle is large , Brownian motion will not be seen.
• Temperature: The higher the temperature, the faster the motion of the particle and hence Brownian motion will be observed.

To see Brownian motion in a liquid, place some wtaer with graphite particle suspended in it into the container.