Brownian motion (named after the botanist Robert Brown) is the presumably random drifting of particles suspended in a fluid (a liquid or a gas) or the mathematical model used to describe such random movements, which is often called a particle theory.
The experiment of Brownian motion in a smoke cell is a classic experiment that gives strong circumstantial evidence for the particulate nature of air.
✓ Smoke cell, incorporating a light source and lens (Whitley Bay pattern)
✓ Microscope, low power (e.g. x10 objective, x 10 eyepieces) and large aperture
✓ Power supply, 0 to 12 V dc
✓ Microscope cover-slip
✓ Smoke source (e.g. paper drinking straw)
Setting up apparatus:
The smoke can come from a piece of burning cord using a dropping pipette or a burning straw (preferably paper). The straw should burn at the top and then be extinguished. The bottom end of the straw should poke into the plastic smoke container.
The cell may need to be cleaned if a waxy or plastic straw is used.
Remove the glass cell from the assembly to clean it. Afterward, push it fully back into the assembly. It may help to wet the outside of the glass tube. You will find it helpful to clean the glass cell after every five to ten fillings to obtain the best results; otherwise the light intensity is reduced.
The cell is illuminated from one side to make the smoke particles visible under the microscope. This is called dark ground illumination. A small piece of black card prevents stray light from the lamp reaching the eye. The lamp is placed below the level of the glass rod in order to minimise convection.
1. Fill the cell with smoke using a dropping pipette and cover it with a glass cover-slip. This will reduce the rate of loss of smoke from the cell
2. Place the cell on the microscope stage, fit the mask and connect to a 12 V power supply.
3. Start with the objective lens of the microscope near the cover-slip. While looking through the microscope, slowly adjust the focus, moving the objective lens away from the cover-slip, until you see tiny dots of light.
4. Watch the particles carefully. Note what you see.
Expected observation to show Brownian motion:
We are expected to see jiggling points of light. The vertical component of the motion causes the bright points to go out of focus and to disappear. The points of light may also have a drift velocity due to large scale convection effects. The bright specks of light do not bounce into each other before changing direction due to collisions with much smaller and invisible air particles. Practical advice
As this is such an important experiment - one of the few to show the 'graininess' of nature and to give strong support to the idea that gas molecules are in constant motion - students should be given plenty of time to set it up and see it clearly.
We know what the students are supposed to see. They may not. Consequently, the students may not ‘see’ what we expect. We expect them to observe jiggling points of light. The vertical component of the motion causes the bright points to go out of focus and to disappear. This will not be obvious to every student. The points of light may also have a drift velocity but we know that this observation is unimportant. The students don’t know that the drift (due to large scale convection effects amongst other causes) is unimportant, and so this may become their major observation. A 'prepared' mind helps the scientist to see.
Once the students ‘know’ what to look for, it is useful to repeat the experiment – they’ll see the expected effect second time around.
The bright specks of light do not bounce into each other before changing direction. Why?
Ask students to note down an explanation of the observations. Discuss what everyone...