Return of the bubbles

Physics of the Mundane

Adam Johnston

“Blow a soap bubble and observe it. 

You may study it all your life and

draw one lesson after another in

physics from it.”

Lord Kelvin

FOCUS FOR THE DAY:

You have seen this all before, but it is time to return to the bubbles and play with them some more.  What observations do you make now that you did not make before?

Specifically, consider the following:

C   Observe a bubble as it sinks into a cloud of carbon dioxide.  (This is produced either with dry ice or with a chemical reaction.)  Based on what you know about buoyancy (a.k.a. Afloatyness@), compare the carbon dioxide to the air inside the bubble.

C   Again, observing the bubble as it sinks into the carbon dioxide, note any change in its size and buoyancy, and explain how these properties coincide.

LOOKING FORWARD TO THINGS YET TO COME (OPTIONAL EXTENSION):

As you continue to play with the bubbles, and as time and patience allow, you might continue on with some other ideas.  Both sound and light can be described as waves.  Later we will investigate sound behavior in closed tubes that exhibit resonance, and we will also witness interference patterns of light as it passes through narrow openings.  Both of these are the result of the wave properties that both sound and light exhibit.

Soap bubbles and oil slicks behave as thin films.  As such, light waves reflect off both the front of the film, as well as off the back of the film.  These two reflections interfere with one another, causing certain waves to be destroyed (as in the dark spots of the interference pattern you saw emit from a narrow opening) and certain waves to be enhanced (as in the bright spots of the interference pattern you saw emit from a narrow opening).  This is known as thin film interference.

Just as sound can be made from many different wavelengths (pitches), light is also comprised of multiple wavelengths.  Each different visible wavelength constitutes a different color.  (All of the colors of the rainbow together create white light.)  Recall how each different length of tube enhanced one particular wavelength of sound. In soap bubbles, each different thickness of a bubble wall enhances one particular wavelength, or color, of light; and, at the same time, the same thickness will negate one particular wavelength of light through destructive interference.  This is done via the process of thin film interference.  In a following lab, we will adjust sound tubes= sizes to values that tune them to a particular wavelength of sound.  (Imagine someone playing a trombone, and this trombone’s length is changed in order to change the pitch that emanates from the instrument.)  Similarly, the thickness of bubble walls are “tuned” to specific wavelengths of light B about 0.0000005 meter!

To introduce yourself to the beauties of thin film interference, observe the bubbles and the colors within the bubbles, and think about the following:  What is the relationship between the thickness of the bubble and the color of the film?  (Note: Red is the longest wavelength of visible light, and violet is the shortest wavelength of light.)  As a bubble evaporates completely, it can actually begin to turn invisible.  What do you think is happening, in terms of the thin film interference?  Where is a soap film the thickest?  Describe the pattern of colors in a sheet of a soap solution. (You can use a cup, string or straw arrangement to create this.)  Explain them.