Imagine this: you’re walking down a hallway, panes of glass to your right and stretches of dull, beige-colored wall to your left. Aimlessly passing your gaze to the floor, you notice a wonderful sight—a rainbow.
Light as a continuous spectrum
The phenomenon that you have just observed is called dispersion; the white light from the sun was dispersed by the glass pane into its constituent colors. The variety of colors that make up white light are not distinct, rather they are a part of a spectrum—the electromagnetic (EM) spectrum, named so because waves of electromagnetic radiation compose it.
The portion of the EM spectrum that can be visually detected by our eyes is aptly classified as “visible light”. Our eyes can detect the whole, continuous spectrum (“white light”) or a distinct frequency of EM radiation that composes it (e.g. “red”, “lavender”, “smaragdine” etc.).
Dispersion is one way of discovering that white light is a spectrum—thin-film interference is another, more complex way of uncovering white light’s hidden truth.
What is thin-film interference? It is the natural phenomenon where waves of light reflected at the boundaries of the thin-film interfere with each other to create what our eyes perceive as “new” light waves. A thin film can be any material (more often than not, either a liquid or solid) whose thickness is, well, thin. How thin? On the order of nanometers to microns, comparable to the magnitude of the wavelength of light.
Have you ever walked down the side of the road and seen the image below?
Although there are many ways we can observe the phenomenon, this is one very common form of thin-film interference and what it can look like. The diesel fuel in this case is the thin film. The beautiful rainbow gradient pays homage to the rainbow we saw and examined earlier; no colored lights are being shone on the film at all, so white light must be directly involved in making this pattern. White light from the sun strikes the oil, the individual colors disperse, and the reflected waves interfere with each other to create the beautiful colors. Keep reading to find out the science behind the beauty of thin-film interference.
First, it’s important that we assume light to be a wave (1), and that our light source is visible light.
Let’s suppose there are three mediums, one atop the other: air, a very thin film of oil, and asphalt. As light travels from the air to the oil, part of the wave is refracted, or bent, and transmitted through the oil and part of the wave is reflected at the boundary (Ray 1). The refracted wave of light travels through the oil, eventually reaching the asphalt, where yet again part of the light is reflected at the oil-asphalt boundary (Ray 2).
It is these reflected light waves (from the picture: Ray 1 and Ray 2) that travel through the air, interfere with one another, and give the appearance of the oil having different colors.
Because all EM waves have unique wavelengths, the thickness of the thin film of oil is very important in determining which light waves are reflected and interfere with each other. It is the varying thickness of the oil film that gives rise to the different colors so characteristic of this common sight.
As of right now, you might be thinking that thin-film interference has no significant influence on life or the betterment of society. It is simply something beautiful to appreciate. That notion is so wrong. So wrong, in fact, that there is a field of science dedicated to the studying of thin films, dubbed thin-film optics!
Sure there are many aesthetically pleasing occurrences of the phenomenon such as soap bubbles and oil slicks, but there are equally as many important economic, social, and evolutionary applications of thin-film interference.
The concept of thin-film interference can be a hard one to grasp, and the mathematics behind it can be even more challenging. Nonetheless, my vision for this article was to shed light (pun intended) on a topic that I see very few articles about. It’s a paradox; a topic so rarely talked about yet so omnipresent in our daily lives deserves this exposure, and that is my hope for this article.
I encourage you to leave comments, discuss with others, and ask questions. I’ve included some links below to learn more on the topic.
(1) Allain, Rhett. “Is Light a Wave or a Particle?” Wired, Conde Nast, 3 June 2017, http://www.wired.com/2013/07/is-light-a-wave-or-a-particle/.
(2) “Light Absorption, Reflection, and Transmission.” The Physics Classroom, http://www.physicsclassroom.com/class/light/Lesson-2/Light-Absorption,-Reflection,-and-Transmission.