As I pondered what to write about for this blog post, I began to consider a topic that we touched upon in Physics B: thin film interference. I really never understood thin film intereference last year... as a matter of fact, there was a thin film intereference free response question on the AP last year and I just stared at it in a very confused manner. So, I figured I would take a second look at it this year.

Thin film inteference happens when the upper and lower boundaries of a thin film, such as a soap bubble, both reflect light waves and their interference becomes a new wave. This occurs because the film is so thin, making the boundaries of the substance incredibly close to each other. For thin film diffraction to occur, the medium must have a thickness in sub-nanometers or microns. Interference is constructive (or creates a new wave) when the difference of optical path is an integer multiple of the light's wavelength.

With a monochromatic light source, the interference appears as light and dark bands... But, when the source is broadband, like the sun, the interference appears as multiple color bands - hence the rainbow glare on bubbles and oil on the road.

Oil is slightly different than soap interference. Oil sits atop a layer of water, and has an index of around 1.5. Constructive interference occurs when the difference of optical path is an integer multiple minus 1/2 multiplied by the wavelength.

There you have it... thin film diffraction in its finest. I'm glad I understand it after I needed to use it in Physics B! At least I learned it eventually, and now you all know why oil on the road looks rainbow! Until next time, Fizzix community, until next time.