What experiments demonstrate Brownian motion?

Robert Brown's pollen experiment and Jean Perrin's experiment with colloidal suspensions are key demonstrations of Brownian motion.

Robert Brown, a Scottish botanist, first observed Brownian motion in 1827. He was studying pollen grains suspended in water under a microscope when he noticed that the tiny particles in the pollen grains were moving in a random, haphazard manner. This movement was not due to water currents or the pollen being alive, as he observed the same motion in inanimate particles like dust. Brown's observations were the first recorded instance of this phenomenon, which was later named Brownian motion in his honour.

In the early 20th century, French physicist Jean Perrin conducted a series of experiments that provided empirical evidence for Albert Einstein's theoretical explanation of Brownian motion. Perrin used a suspension of gamboge particles (a type of resin) in water and observed their erratic motion under a microscope. He was able to calculate the displacement of the particles and found that it matched Einstein's predictions. Perrin's experiments were crucial in confirming the existence of atoms and molecules, as Einstein's theory was based on the premise that Brownian motion was caused by the random movement of molecules in a fluid.

In modern times, Brownian motion can be easily demonstrated in a classroom setting using a simple setup. A drop of milk is added to a glass of water and the mixture is observed under a microscope. The tiny fat globules in the milk can be seen moving randomly, providing a visual demonstration of Brownian motion. This experiment can be performed by students to better understand the concept and its implications in various fields such as physics, chemistry, and biology.