Visual Magic

Colors

As mentioned in the last blog post, I will be discussing the science behind the cool colors of bubbles in this blog post.

Spend 30 seconds trying to describe the bubble’s color.

It is impossible to ascribe a single color to a bubble, as a bubble has an iridescent nature; in other words, the color shifts wildly from different angles. The same iridescence appears on numerous other objects in nature; for example, a gasoline spill’s swirling rainbow patterns on its surface, or a butterfly wing’s shimmery color.

While deceivingly simple, there is a surprising amount of science behind the colors that appear on a soap bubble. In this post, I will be attempting to provide an easy-to-understand explanation of the iridescence of a bubble. The post will be broken down into subsections to help with this explanation.

Light

To begin with, we investigate the driving force behind color: light. Light is an “electromagnetic wave”, meaning that when it travels through space, it takes on this form:

Image Credit: http://www.amateur-radio-wiki.net/index.php?title=Electromagnetic_wave

Different colors of light have a unique “fingerprint”; namely, the distance between the peaks, also known as the wavelength. The longer the wavelength, the redder a color is. On the other hand, a shorter wavelength means a color is more on the violet end of the spectrum.

Furthermore, the amplitude of the wave, or the distance from the center of the wave to a peak, is responsible for the brightness or intensity of the color. The greater the amplitude of a wave, the brighter and more intense the color.

Reflections

When light hits a transparent surface, some of it is reflected off, while some passes through. This is apparent when we look at a glass window. A fragmented ghost-like reflection appears; only some of the light is reflected – the rest enters the glass unhindered.

The same happens when light encounters a soap film. Some of the light will venture into the soap film while some will be reflected directly.

Wave Interference

Waves can interact with each other. The interaction of waves is known as interference; peaks in opposite orientations traveling towards each other will cancel out (destructive interference),

Image Credit: http://physics.bu.edu/~duffy/py105/WaveInterference.html

while peaks in the same orientation traveling toward each other work together (constructive interference) to magnify the amplitude of the wave.

Image Credit: http://physics.bu.edu/~duffy/py105/WaveInterference.html

In the case of a soap bubble, interference occurs when a certain color of light is partly reflected and passes through.  

When part of the incoming light is reflected off of soap water, it will exit in the same orientation as it came in.

On the other hand, the light that passes through and reflects off the inner surface will travel an extra distance that can change the output orientation. If the total distance traveled by this wave, equal to twice the thickness, is half the wavelength, then the wave will exit with an inverted orientation that it entered with. When this meets the initially reflected wave, the two waves will cancel out because they have opposite orientations. 

Image Credits: https://en.wikipedia.org/wiki/Talk:Soap_bubble

In contrast (no pun intended), if the distance that the wave that enters the soap film travels is equal to one wavelength, then it will be in the same orientation as the reflected wave. Thus, when this meets the reflected wave, the two waves work together to create a more vibrant color.

Image Credits: https://en.wikipedia.org/wiki/Talk:Soap_bubble

There are additional phenomena such as phase-shifting that contribute to bubble color, but those are beyond the scope of this blog post.

To demonstrate this property, I set up my softbox and camera again.

This interference is apparent in the following images and video I took:

Video Link

With the film varying in thickness throughout from gravity causing soap water to bunch up towards bottom, different colors are constructively and destructively interfered due to the different thicknesses causing the bands of colors.

This phenomenon of interference causing hypnotic colors appears everywhere. From a color-changing car wrap to a peacock’s feathers, iridescent interference can be observed everywhere. All it takes is keeping your eyes open, both literally and figuratively.