A plot of 2D Brownian motion
The above plot shows the randomness in movement of particles.
Particle changes its direction instantaneously giving rise to zigzag motion called as Brownian movement.
Einstein, in his papers published in 1905 gave the explanation for this which states that matter is made of particles.
The credit for indirectly observing the existence of molecules first goes to Robert
Brown, a Scottish botanist. In 1827, Brown observed under a microscope that pollen
grains, which were of the size 10−5 m, when suspended in water appeared
to perform random motions. He initially thought that the pollens were alive! But
more observations, with pollens of dead flowers, and pollens boiled in water, also
showed similar motion. This type of random motion of particles is termed as Brownian
motion. Zigzag motion of small particles can be seen in our everyday lives also.
For example, small ash particles emitted from a coal fire can be seen to undergo
zigzag motion in the air, dust particles can be seen to perform a similar motion in
a strong beam of sunlight. Brownian motion increases or decreases with various parameters.
Einstein developed the theory of Brownian motion in 1905. He explained that random
motion of pollen grains in water was due to the motion of water particles themselves.
Thus he observed that Brownian motion is the result of the continuous impact of
the molecules of the liquid on the suspended particles. Further, though it was known
that the matter is made up of molecules, the discovery of Brownian motion and Einstein’s
explanation of the same gave the first direct evidence of the presence of small
physical particles, molecules, undergoing random motion.