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Thin Film Interference: The Movie

  1. Oct 11, 2011 #1
    I put together this little movie as (hopefully) the first in a series.


    My target audience is a second semester class of physics students that have already had a lectures in basic wave optics. When I give this URL to my students, it's with the expectation that they will be able to play it multiple times, and to pause it to see what's happening.

    This describes thin film interference by showing what the waves are doing, how the refractive index matters, when the inverted waves happen, etc.

    It took over a year to put this together in my spare time, using POV-Ray and the voice of one of my students.

    Feedback would be appreciated ... add feedback and comments to the YouTube page ... I might not see them right away if you reply here, but anywhere is fine. (Any press is good press, as they say. :)

    Note that I'm already aware of a few editing issues I had with the piece. I'm interested in whether or not (and how much) this would be useful. While criticism is fine, please be kind. Remember, I spent a long time on this.

    Thanks and enjoy!
    Last edited by a moderator: Apr 26, 2017
  2. jcsd
  3. Oct 11, 2011 #2


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    Very nicely presented with the Greig Peer Gynt mixed in tastefully.
    I particularly liked the little flags, showing phase reversal or non phase reversal - excellent.

    I might suggest that the basic message of how the interference works at different wavelengths could be delivered quite a bit earlier on and in brief, showing how path difference causes a different phase shift for different wavelengths (as you do, later on - but in brief, the first time round).
    If this is followed by the effects, in detail, of different values of refractive index and thickness etc, then students will already have got the principle of the filter operation and can see where the various parameters affect the process. I 'knew the ending of the story' but your students may not see where it's leading in its present form.
    Telling them the important message more than once can only be a good thing!

    Sorry but I don't subscribe to utube so this reply will have to do.
  4. Oct 11, 2011 #3


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    Wow, that's a very impressive work. I do agree w/ sophiecentaur's comment about presenting the basic conclusion both early and late.
  5. Oct 11, 2011 #4

    Andy Resnick

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    Very impressive piece of work- kudos! One question/comment- what are the learning goals for this video? In other words, when students finish watching the video, what will they have learned? Is this video coupled with a lab? How does this fit in with the other clasroom/homework activities?
  6. Oct 12, 2011 #5


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    ezfzx: congratulations! Wonderful educational video. I think it will be useful when viewed along with the classroom activities (maths). Although I am a lover of Greig's music I think it is a distraction here.
  7. Oct 12, 2011 #6
    @sophie: I like those comments, and had considered having a kind of "teaser" at the start, and putting the whole work in the form of an "abstract, body, conclusions" format ... maybe the next revision (there have been 5 already). I'm trying to figure out how to implement a quick version of your idea without rebuilding everything and/or hopefully without asking my narrator to record anymore dialog (which I can do if I have to).

    What exactly would you like to see ... the red/green/blue colors part in a short clip near the beginning? (The nice thing about talking directly with the author is getting to contribute to the product! :) Tell what I can add that would make this useful to you.

    @Andy Resnick: I struggled with what content to add and what to leave out. Originally, the shorter non-narrated version of this piece was built into my PowerPoint slides as part of the lecture. So, my audience had already see the equations and discussed the concepts and now could see a visual representation. This is why I hadn't added the teaser originally, because it wasn't designed to stand alone.

    We do have thin film labs (college classes), and many frames from this video are used as illustrations in the written lab material. The goal of the film is just to describing visually what I've been talking about verbally for years.

    Here are some of the concepts my students had difficulty visualizing that I hoped this film would address:

    • Phase shifting on reflections from higher index materials.
    • Compressed wavelengths within the medium with a higher index.
    • The distance traveled within the film is two the width.
    • Light emerging is a superposition of two waveforms.
    • Different colors represent different thicknesses.
    • The thin film phenomenon is extremely small in scale.

    Now, of course, I'm moving toward something more self contained, but there's no way to guess what a random viewer knows about wave optics ahead of time. It would be nice to have the time to build up a set of "soup to nuts" videos for the entire lecture, but there are so many good videos already out there. I just couldn't find any that described thin films to my satisfaction.

    Then, of course, when you get deep into a project like this, it's easy to have days where you forget what the goal was, so I had a little sticker on my desk reminding me what I was doing.

    The music I chose was literally added at the last minute when my previous selections raised too many concerns about copyrights. With classical music, you only have to ask permission from the producer, not the composer. :)
  8. Oct 12, 2011 #7

    Andy Resnick

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    Nicely done- it's clear you have spent a lot of time and thought on this. As for specific items, I would suggest the following:

    Phase shifting- the basic idea is good (with the flags and the narration about when the refractive index is higher or lower), but the three sine curves show a discontinuity at the interface- this could lead to confusion (does the E field undergo discontinuous changes at the interface?). It also introduces notions of polarization which would obscure your otherwise clear presentation.

    Compressed wavelengths/distance traveled- *very* effective.

    Superposition- again, the basic idea is good but the execution could lead to confusion- since the waves are clearly separated in space (or at least they appear to be separated), why do they interfere? Your animation actually has the light 'curving and merging' together.

    Colors- again, a very effective use of POVray. Initially, the animation is rather cluttered- all those different colored sine waves- but when you took those away and had the colors spatially vary with the thickness, that was *really* great. It may be useful to incorporate angle as well- tilting the film instead of changing the thickness (which is the same thing, but may not appear so. And tilting the film is often how colors are selected, etc. etc.)

    Small scale- If that refers to the slicing-n-dicing section near the beginning, it was a little fast-paced and unclear. I like your use of length scales, and I like how you ended up- your 'box representation', using heights and colors, is extremely well done. Maybe a slower zoom without all the cuts?

    Keep up the good work- I'm sure your students will benefit from this!
  9. Oct 12, 2011 #8
    @Andy Resnick:
    Yeah .. I had a LOT of discussions with my little volunteer focus group about that very point. As my artificial deadline approached (I was trying to get it finished by the end of the summer), that continuity component was dropped as something that would take a lot of time to program with very little direct benefit in the short term.


    The original story boards had the waves begin together and pull apart into components, but that scene was dropped initially because I didn't want to go too far down the superposition conceptual trail. By the time we talk about thin films (TF) in class, we'd already covered superposition to death, so my audience should understand what I'm doing.

    But I do agree that it would be a good element to incorporate next time around as I move toward self-contained content.

    The original ORIGINAL vision of this video was to have surface waves, like a simulated wave tank, instead of individual waveforms floating in space ... but it was a little tricky and messy to look at. Maybe down the road ...

    The plan was to make use of an object in POVRay called an "isosurface", which is basically a shape that is defined by an equation. I wrote a little bit of code that associates the equation with the isosurface AND a color map representing what the TF pattern would look like for the various surface thicknesses. I wanted to have a complex 3D shape ripple across the inside surface of the bubble and have the outside colors map it. The prototypes looked FANTASTIC and I was eager to build it in, but render times with isosurfaces take FOREVER compared to the rest of the project. (Complex reflection and refraction that make the picture look ultra-cool takes a long time to calculate in high resolution even with a couple of quad-core systems working in tandem ... I need a render farm!)

    I tried every programming trick I knew to optimize the code for a faster tender, but I was out of time. I'll build it in next for sure, though, and start the render at the beginning of the summer instead of waiting until the end.

    I'm not sure what you mean ... but I think you mean reducing the number of slices of the bubble? We (my guinea pigs and I) had a lot of discussion about that in class too. The feedback I got was that watching the slicing continue to go and go and go demonstrated just HOW small things were getting ... like, "Is it small enough yet? No, it's still going! Is it small enough yet? No, it's still going!" I do something similar with a scaling video I'm putting together that was inspired by the famous "Powers of Ten" video. Enduring the many slices is MEANT to demonstrate the scale. That was the intention anyway. You should see all the other clips I rendered and discarded ... failed prototypes constitute the bulk of the development time as you can imagine.

    Originally I wanted to have familiar objects nearby growing in size as the slicing got smaller ... crickets, paperclips, snowflakes ... but it just looked so busy and took so much time.

    When I show the film in class, I point out all the short comings (I'm my own worse critic), and explain what "really" happens, and am building a little "punch list" of items to revised for the next rev.

    I really only get the summer to work on these, and with virtually no support from my school, I just copyright them in my name. (Though I had considered using some service like kickstarter.com or something like it to maybe get some contributions ...)

    I'm hoping its not a waste of time. Justifying my time on these projects to my wife is the biggest hurdle. I tell her that maybe Pixar will notice me.

    After all, isn't that why we became teachers? Money? Fame? Babes? OK, maybe I was misinformed. ;)
  10. Oct 12, 2011 #9

    Andy Resnick

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    I totally understand- you have to make choices (this applies to everything you wrote above that I clipped out). Again- the video is a really nice piece of work, and I truly hope the students respond positively to it.

    Love it! All I was saying is that I needed a minute to get synchronized with your video- all the jump-cuts of a bunch of great photos of thin films set a very fast visual pace, maybe the rhythm of the animation was different from the photo montage. Your concept is spot-on.

    :) Seriously- good luck with this project, and please let me know how the students responded.
  11. Oct 12, 2011 #10
    Oh ... my mistake.

    Yeah, I was trying to minimize the material that wasn't mine and get to the stuff that was. My wife complained about it too, but I console myself in knowing my Internet savvy students will simply stop the video if they want to look at any one thing longer. ;)

    If I had my preference, I'd spend about 5 minute explaining the details of each photo ... they are all just that cool. All different materials, different indices ... I especially liked the coins.

    The beginning does run a little rough ... I'll make a note to see if I can get a better flow in the next revision.

    Thank you for your detailed feedback! :)
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