01 / interference ·
where do the colors of a soap bubble come from?
An introduction to light interference through the colors of a soap bubble.
I remember being deeply intrigued, as a child, by the colorful stains on asphalt after a rainy day. How was that possible? Where did those colors come from? I had no idea that a thin layer of oil could produce that effect, much less that it was caused by light interference. So I called it magic, like everything we don't understand.
Now, decades later, I use the same optical phenomenon every day to solve real engineering problems. And I finally understand it a little better (I think).
During my master's degree in Brazil, my advisor proposed a challenge: how could we use the colors of a soap film to measure its thickness? It was the same optical principle, now being used to measure a layer that can be just a few hundred nanometers thick. To put that scale in perspective, 500 nanometers is half a micrometer: roughly one hundred times thinner than a fine human hair.
Let's discuss, then, how these colors relate to the thickness of the film.
01 / light, waves & interference
We can describe light as a wave. Imagine a sequence of peaks and troughs: the distance between two consecutive peaks is the wavelength, represented by the Greek letter λ (lambda). For light of a single wavelength, that distance is associated with color: shorter wavelengths correspond to blue and violet; longer ones, to red. White light, like sunlight, combines many wavelengths. That is why it can give rise to so many colors.
The phase describes where a wave is within that cycle. Two waves can have the same distance between their peaks but be shifted relative to each other: the peak of one can arrive alongside the peak of the other, or alongside a trough. It is like a clock: after one full turn, the hand returns to the same position. For a wave, that turn corresponds to 360°.
When two light waves meet, they add together. If peaks line up with peaks and troughs with troughs, they reinforce each other: constructive interference. If the peaks of one line up with the troughs of the other, they weaken each other and, when they have the same amplitude, can cancel: destructive interference.
02 / two reflections, one film
Let's return to the stain on wet asphalt. A film of oil can spread over water. When sunlight arrives, some reflects from the surface of the oil; some enters the film, reflects from the lower surface and comes back out. Something similar happens in a soap film: light also reflects from both surfaces.
The two reflections reach our eyes after traveling along different optical paths. Light reflected from the lower surface travels an extra path inside the film. The difference between these paths changes how the waves line up, or their relative phase. Reflection itself can also invert a wave, exchanging peaks and troughs.
Because white light contains many wavelengths, the two reflections do not add in the same way for all of them. At a given thickness, some colors are reinforced and others weakened. What we see is the combination of what remains: that is where the film's colors come from.
03 / how does thickness change the colors we see?
A thicker film changes the path of the light that enters it before reflecting. This changes the relative phase between the two reflections and, therefore, which wavelengths are reinforced and which are weakened.
Move the thickness slider. Watch the film diagram, the three curves and the color mixture: the same thickness can reinforce one color and weaken another.
04 / why do the colors repeat?
Phase goes around, like the hand of a clock. As thickness increases, the waves pass through alignments that repeat: reinforcement, cancellation and reinforcement again. That is why, for a single wavelength, a strong reflection can correspond to more than one thickness.
Each wavelength completes these turns over a different interval of thickness. Two thicknesses can reflect green with the same intensity but reflect red differently. Comparing several colors helps distinguish these possibilities. To measure thickness, we compare the observed intensities with those predicted for different thicknesses, accounting for the illumination and the camera.
In the photograph from our experiment, the bands indicate that thickness varies across the film. As it changes, so does the combination of reflected colors. The angle between the light and the film also influences that combination (which is why the colors change when we move a bubble). In the interactive, we keep that angle fixed to observe only the effect of thickness.
05 / can interference become a measuring tool?
The colors and the way different wavelengths interfere carry information about a path traveled by light. How can we use that information to measure dimensional changes at the nanometer scale?
In future posts, we will explore how light can be used to measure. I will also share optical phenomena, engineering applications and experiences from my research, connecting science in everyday life with the work we do in the lab.
The next post will be about Schlieren: how can we see air? In the meantime, here is this week's music recommendation: Colours - Donovan.
next / can we see air?
