(A sphere, for a given volume of air inside the bubble – the bubble size – has the smallest possible surface area so uses the least possible bubble mixture)
5. Colours in bubbles (for older students)
Bubbles appear to have swirly colour in them.
White light is separated into colours as it reflects from two surfaces of a thin film (eg a bubble or an oil slick). Each colour reflects from both the outer and inner surfaces of the bubble film. Because the bubble has some thickness, this light is slightly out-of-phase and the two reflecting rays can interfere. If the interference is constructive the colour becomes more intense, if it is destructive the colour effectively cancels out and we can’t see it. As the bubble gets thinner (eg under gravity) the two reflected rays of light coincide and cancel and the bubble loses its colour. Bubbles are a cheap and very easy way to demonstrate that light is made up of colours and that colours can interfere.
The alternating bands of light and dark on this soap film are actually bands of color, produced by the reflection and interference of light waves. The colors depend upon the film's thickness. The film shown here is thinnest at the top, becoming thicker toward the bottom. As the film's thickness changes, the colors also change, forming regular bands
If you let a bubble hang from a bubble wand for a while, the interference colors begin forming horizontal stripes - because the bubble film is thicker at the bottom than at the top, forming a wedge shape. As the bubble drains, the wedge of bubble solution gets thinner and thinner. The black film which then appears at the top of the bubble is a harbinger of an upcoming disaster. The bubble is now so thin only a few moments remain until. . .POP!
Get the students to observe and describe what happens to the bubble colours if you let the film stay on one of the bubble wands and slowly fall under gravity.
Surface Tension Demonstrations – Worksheet