My unusual method of teaching thin film interference

I would NOT assign a soap bubble problem in the first
day or two of covering thin films -- why not?

Got a nice note the other day from William Haskell, a new physics teacher in New York.  He was reading my 5 steps book about thin films, and was initially confused.  I didn't mention phase changes at all! 

I said, in a thin film, the beam of light that passes through the film goes an extra distance 2t, where t is the thickness of the film.  So constructive interference occurs when that extra distance is a whole number m of wavelengths.  That's where the equation 2t = mλ comes from.

When I teach thin films, I intially very deliberately fail to mention phase changes.  I'm just following the general principle of teaching physics:  start simple, add complexities later.  I want the students to become comfortable with the idea of interference in a thin film, with the relevant equation, with the meaning of m, with the types of problems that might be asked.  

So, everyone starts by solving thin film problems with no phase changes.  Then I introduce the idea of a phase change when light reflects off of a higher-index material.  We simply say, "if there is exactly one phase change, then switch the conditions for constructive and descructive interference."  

If I had made that statement on the first day of thin film study, I would have gotten blank looks.  By waiting until the class became comfortable with the no-phase-change approach, I ensure that they have the context for this complication.

Thanks for the note, William.  I will be giving an AP summer institute at Manhattan College this summer in the first week of August -- come on and join us!

GCJ