Why Do Soap Bubbles Look Rainbow-Colored?
β The Colors Come from a Different Trick Than a Rainbow
On the surface of a soap bubble, you'll see a shimmering, shifting rainbow pattern, much like an oil film floating on water. You might assume "a soap bubble is just a kind of rainbow" β but its colors actually come from a completely different mechanism than the rainbow in the sky. A rainbow's colors appear because light "splits apart." A soap bubble's colors appear because light "overlaps."
Blow a soap bubble, and a rainbow-like color pattern appears on its surface. Because you see reds, greens, and purples lined up, just like in a rainbow in the sky, it's tempting to assume it's "the same mechanism."
But look closely at a soap bubble's colors, and you'll spot a crucial difference from the sky's rainbow. A rainbow is a fixed-shape band where the colors always line up in the same order. A soap bubble's colors, on the other hand, keep swirling and changing shape every time the film wobbles.
This difference tells us that the very principle behind how the colors are created is fundamentally different.
Inside a raindrop, light refracts at a slightly different angle for each color and splits apart (disperses), and this creates the rainbow's colors.
Light reflected off the front and back of a soap bubble's film overlaps (interferes), and only the colors that reinforce each other become visible.
"Color made by splitting" versus "color made by overlapping" β let's look at this difference step by step.
A Rainbow Is "Dispersion," a Soap Bubble Is "Interference"
As explained in detail in another article, a rainbow occurs because when light refracts inside a raindrop, each color (wavelength) bends at a slightly different angle. This is called dispersion. A single beam of white light "splits" into many beams of different colors β that's how a rainbow works.
A soap bubble's colors work completely differently. A soap bubble's film is a soap-water film so extremely thin that it approaches the wavelength of visible light. When light hits this film, part of it bounces off the front surface, and the rest passes through the film and bounces off the back surface as well.
When the "light reflected off the front" and the "light reflected off the back" travel in the same direction and overlap, the crests of the waves reinforce each other where they align, and cancel out where a crest meets a trough. This is called light interference.
A soap bubble's colors come from light "overlapping."
The Film's Thickness Decides Which Color You See
Which color reinforces depends on the film's thickness. Where the film is slightly thicker, one color (say, blue) reinforces; where it's slightly thinner, a different color (say, red) reinforces.
A soap bubble's film gradually drains downward under gravity, so its thickness varies from place to place. As a result, a swirling pattern of colors appears on the film's surface, corresponding to these thickness differences. As the film wobbles, the thickness distribution shifts too, so the color pattern keeps changing constantly. That's why, unlike the sky's rainbow, a soap bubble's colors never stay still.
Just Before It Pops, a Soap Bubble Turns "Transparent Black"
Just before a soap bubble pops, part of the film can turn into a dark patch that barely reflects any surrounding light. This is thought to happen because the film becomes far thinner than the wavelength of light, so the front and back reflections cancel each other out almost completely. In other words, it's not that the color disappears β it looks black because every color cancels out at once.
Try It Yourself
- Make a soap bubble with soapy water (or stretch a film of soap solution across a ring) so that it stays intact as long as possible without popping
- In a bright spot, watch the color pattern on the film's surface for several tens of seconds
- Confirm that the color pattern gradually shifts in a swirling motion
- Just before it pops, look for a patch that appears to turn dark
The moving color pattern is proof that the film's thickness keeps changing over time. This is something you will never see in the sky's rainbow.
Summary
A soap bubble's rainbow colors look similar to a sky rainbow but come from a completely different mechanism. A rainbow's colors come from "dispersion," where light splits apart by color. A soap bubble's colors come from "interference," where light reflected off the front and back of the film overlaps. Because the visible color changes every time the film's thickness changes, a soap bubble's pattern keeps moving constantly.
A soap bubble is not a tiny rainbow.
It's an entirely different kind of art, made from light overlapping with itself.
There are other phenomena that create color purely through interference, without any pigment. For why an opal shines in rainbow colors, see this article.
Want to know more? β Terms, numbers, and links to the textbookWe label each section by level, from middle-school science to topics still being researched
- MSCovered in middle-school science
- HSCovered in high-school "Physics"
- HS+Covered in high-school "Physics," or treated as advanced/sidebar content in textbooks
- Univ.Not covered in high school β a university-level specialist topic (optics)
- ResearchNot yet settled "textbook fact" even at university β something researchers are actively studying
MSTerms: Words behind a soap bubble's colors
- Dispersion: The phenomenon where light refracts at a different angle for each color (wavelength) and splits apart. This is how a rainbow works.
- Interference: The phenomenon where multiple waves overlap and either reinforce or cancel each other.
- Thin film: A film so extremely thin it approaches the wavelength of light.
HSChecking with a formula: Just how thin is a soap bubble's film?
Let's actually calculate the film thickness needed for a given color (wavelength) of light to reinforce.
Film thickness Γ refractive index Γ 2 οΌ (reinforcing order οΌ 0.5) Γ wavelength
| Film thickness | In nm (nanometers, one-billionth of a meter) |
| Refractive index | For a soap film, close to water's, about 1.33 |
| Reinforcing order | An integer: 0, 1, 2, and so on. The thinnest case is 0 |
| Wavelength | The value corresponding to the light's color, in nm |
The "οΌ0.5" appears in the formula because the crest-and-trough position of the light wave shifts by half a wave between the front-surface reflection and the back-surface reflection. This is explained in detail in the HS+ section below.
Let's find the thickness at which green light reinforces in the thinnest case (order 0).
| Refractive index Γ 2 | 1.33 Γ 4 οΌ 5.32 |
| Wavelength Γ· (indexΓ2) | 550 Γ· 5.32 β 103.4 |
| Film thickness (thinnest case) | Approx. 103.4 nm |
1 nm (nanometer) is one-billionth of a meter. This tells us that green interference first occurs at a mind-bogglingly thin scale β roughly 1/1000th the thickness of a human hair.
Increase the order by one, and the same color reinforces at a different thickness too.
| Right side at order 1 | 1.5 Γ 550 οΌ 825 |
| Film thickness (order 1) | 825 Γ· 2.66 β 310.2 |
| Film thickness (order 1) | Approx. 310.2 nm |
So the same green color can reinforce at multiple thicknesses β around 103 nm and around 310 nm. Because the film's thickness varies slightly from place to place, a variety of color patterns appear across the bubble's surface.
β» An actual soap bubble's film thickness varies with location and time; the figures here are just one example of the conditions under which a given color reinforces.
HS+Why Is "οΌ0.5" Needed?
Light has a property where, when it reflects at a boundary going from a lower-refractive-index material to a higher one, its phase shifts by half a wavelength. For a soap bubble's film, this shift happens at the front surface (air β soapy water), but not at the back surface (soapy water β air). The "οΌ0.5" term is needed in the formula to capture this shift, which occurs on only one side.
Univ.Thin-Film Interference Is Also Used in Everyday Technology
The principle of thin-film interference is also applied in anti-reflective coatings used on camera lenses and eyeglasses. By precisely tuning the film's thickness, you can cancel out reflection at a specific wavelength and let more light pass through the lens. A similar principle to thin-film interference is also thought to be involved in "structural color" β color produced not by pigment but by microscopic structures β as seen in the wings of morpho butterflies and peacocks.
ResearchWhat's Still Not Fully Understood
- Exactly how a soap bubble's film stays stable for a certain amount of time and then thins out and pops β the process of drainage and thin-filming β is a topic still actively studied in a field called soft matter physics. Subtle shifts in the film's surface tension (a phenomenon called the Marangoni effect) are thought to be involved, but building an accurate predictive model is said to remain difficult.
- Research is also underway on technology to artificially reproduce structural color more stably. Because it can produce vivid colors without pigments, it's drawing attention as a low-environmental-impact coloring technology, though challenges such as durability reportedly remain before practical use.
- Accurately predicting at the molecular level how a soap bubble film's lifespan changes depending on the type of detergent or additive is also one of the research topics in interface science.
Even the everyday behavior of a soap bubble opens onto the deep, still-unfolding world of thin-film physics.
Links to the Textbook (by Level)
| Level | Subject/Unit | Where in this article |
|---|---|---|
| MS | Science γ» Properties of light | Basic terms: dispersion, interference, thin film |
| HS | Physics γ» Wave interference | Calculating film thickness from the thin-film interference formula |
| HS+ | Physics γ» Reflection and phase of waves | The phase shift on reflection (meaning of οΌ0.5) |
| Univ. | Optics γ» Materials science | Anti-reflective coatings, structural color |
| Research | Soft matter physics (ongoing research) | Thin-film drainage and stability, artificial structural color |
- Physics textbook coverage of thin-film interference (light interference).
- Optics textbook coverage of phase shift on reflection and anti-reflective coatings.
- Soft matter physics research literature on soap-film drainage and stability (the Marangoni effect).
- Research review on structural color (color produced by microscopic structures, such as morpho butterfly wings).
- Interface science textbook coverage of surfactants and film stability.
β» The refractive index, wavelength, and film-thickness figures are representative estimates. An actual soap bubble's film thickness and color appearance can vary depending on observing conditions.
β»This article is a general-audience science explainer. When playing with soap bubbles, take care not to swallow the solution or get it in your eyes, and please check the product's usage precautions.