Bubbles are perfectly shaped, fleeting, and abundant in transient colour, and they perfectly capture the spirit of childlike wonder. They also demonstrate a wide range of helpful scientific concepts, such as surface tension, surface area to volume ratios, and interference colours.
How do soap bubbles come into existence?
- A soap bubble is made up of two layers of soap molecules and a very thin sheet of water.
- The soapy water film encircles a bubble of air.
- When soap molecules move to the inner and outer surfaces, they thrust their water-attracting ends outward and their “heads” inward.
- Soap molecules have two ends, one of which repels water and the other of which attracts it. The bubble would spontaneously disintegrate into tiny water droplets if there were no such molecules on the surface.
- Surface tension causes the bubble to be spherical, which is the shape that gives the minimum surface energy – the lowest ratio of surface area to volume.
What causes the colour of soap bubbles?
The soap bubble’s outer surface and interior both reflect light.
- One part of an incoming light ray hits the bubble’s outer surface and is immediately reflected, while the other part is transmitted into the soap film. This transmitted light ray is returned to the outer surface of the film after passing through its inner surface. It moves in the same direction as the ray that was immediately reflected when it exits the bubble, making it parallel to that ray because it moves in the same direction.
- The second ray will partially cancel out the reflection of the first ray if the wavelengths of these two light rays are reflected in an “out of phase” manner. The intensity of the colours is diminished due to a phenomenon known as destructive interference. The two reflected rays will enhance one another, though, if their wavelengths are “in phase.” Constructive interference is the term for this.
- As opposed to the outer surface of the bubble, which only reflects light, the inner surface of the bubble also reflects light farther than the outer surface. Depending on the additional distance a transmitted-and-reflected ray travels, some wavelengths will interfere negatively while others constructively. The additional distance the second ray must travel (through the film and back) before rejoining the first ray determines whether the reflected rays are in phase or out of phase with one another. This distance is influenced by the film’s thickness and the angle at which the light is incident.
- Different colours, each corresponding to a different wavelength, make up white light. The additional distance the ray must travel varies with the film thickness. When the total additional distance equals a particular wavelength of light, interference is constructive; when it equals half that wavelength, interference is destructive. In other words, if white light hits a bubble, the film will reflect the light of a particular hue, and this hue will vary depending on the film’s thickness.
A soap bubble’s iridescence, which appears to contain a variety of shifting colours, results from light striking the bubble at different angles. The path length varies depending on the angle at which the light is incident, resulting in different path lengths for internally and externally reflected rays at various locations on the bubble. This means that different colours can be seen even when the soap film is of a uniform thickness. Compared to light entering at a wider angle, light entering directly takes a shorter path through the bubble. This enables different wavelengths to experience constructive and destructive interference, which results in the perception of various colours.
Fun Fact Time
- Longest Bubble. With a bubble that is more than 15 metres long, David Stein currently holds the Guinness World Record.
- Soap Bubbles in a Group. The swirls take on a variety of lovely hues. Many times, the swirls will turn black just before bursting.
- Changing Shade. The thickness of the film affects the colours of a bubble. As a bubble dries out, it gets thinner and thinner until it finally pops. A shift in overall colour can be seen, as the bubble’s surface film gets thinner and thinner. Thick walls cancel out longer wavelengths in the red range. Yellow wavelengths are cancelled out as the bubble film becomes thinner. Greenlight is lost as it becomes even more transparent. Even shorter blue wavelengths in the range vanish beyond this point.
Also published on Medium.