Why Are Soap Bubbles Shiny? Understanding Thin Film Interference
Tilt a soap bubble in the light and you'll see a rainbow rippling and swirling across its surface. Where do these colours come from, and why do they keep shifting?
What Is Thin Film Interference?
To understand why bubbles are so colourful, it helps to think of light as a wave. A soap bubble can be thought of as having two surfaces: an outer air-soap boundary and an inner soap-air boundary. When light hits the outer boundary, part of it reflects back, and part of it refracts into the soap film. That refracted ray then hits the inner soap-air boundary where, once again, some light is reflected while some is refracted.
The ray that reflects off the inner surface eventually re-emerges out the bubble, travelling in the same direction as the ray that reflected straight off the outer surface. Both waves make their way toward the observer's eye. However, one of the waves reaches the eye a little later, since it makes an extra journey through the soap film before reflecting back out of the bubble. The rays that emerge out of the bubble interfere with each other as they reach your eyes. In-phase waves (waves whose crests and troughs line up) reinforce each other through constructive interference, creating a wave with greater amplitude; out-of-phase waves (waves whose crests and troughs don't line up) weaken each other through destructive interference, causing the final wave to have decreased amplitude. The colour that is observed depends on the wavelength of the light wave that eventually reaches the observer's eyes.
Factors Affecting the Thin Film Interference Pattern
Thickness of the film: The thicker the film, the farther the refracted ray travels before reflecting out of the bubble. Due to this, the delay between the waves is greater. The colors on a bubble are always moving and changing as a bubble thins over time. Just before it pops, the film often turns completely dark and see-through; this happens because the film has become much thinner than the wavelength of visible light, so the path difference between the two reflected waves is nearly zero.
Type of light: If the light hitting the bubble is monochromatic, the pattern on the bubble after interference will show bands of light and dark. This is because when the incident light is only of a particular wavelength, there is a smaller range of light and dark values of that color that can be formed after interference. On the contrary, white light would result in the most colourful bubble as it will have the greatest variation of wavelengths after interference.
Angle of viewing: The angle you view the bubble from also changes the interference pattern. As the angle of incidence increases the ray bends closer to the plane of the film, and the optical path difference between the two reflected waves gets shorter. A shorter path difference means the delay between the waves is smaller, so the interference pattern shifts toward shorter wavelengths. This is why a bubble may look blueish from the side-on.
Where Else Can You See Thin Film Interference?
Similar patterns of swirling colors can be seen on any surface that has two refractive boundaries. For example, an oil slick on a puddle could also look shiny for the same reason. In tech, thin film interference is used to create anti-reflective coatings. The thickness and material of the coating are chosen so that the wave reflecting off the outer surface and the wave reflecting off the inner surface are out of phase, destructively interfering and cancelling each other out.
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