Light Reflection, Refraction, & Diffraction

I hope it’s not getting old, but I would like to say that for those that have never attended a modeling workshop, although I hope this series of blogs peaks your interest, in no way should they be viewed as a replacement. Compared to the first set of workshop, I will not be posting links to the materials used as ASU and the AMTA have chosen to keep them under lock and key. If you are offended by that, I’m sorry, but they have chosen to restrict some materials, and I am in no place to challenge them. The basic thought process is that there research has shown that one best learns through doing, not through being told.  Therefore, the best way to learn how to implement modeling is to experience modeling for yourself, not to read about it.  In short, if you just read how to model, try to use it and find it ineffective, it could very well be you implemented it incorrectly.  Go to the workshop, you’ll thank me.  For those readers that have attended the Mechanics workshop, this may help you sort through the materials to which you have access, but again, you might want to try to get to one of these if you can.

2-D Wave Motion

We begin this portion of the unit by putting together ripple tanks.  Although these contraptions clearly show the behavior of water, I think there is a much cheaper solution.  By no means do I want to go full scale $2 whiteboard here (I mean no disrespect by that comment, I think that blogpost might be one of the greatest I’ve ever read), but I don’t see why you need that much complexity.  Why not just use a cafeteria tray, your finger and/or a ruler as a wave generator, and some simple objects (or even the edges of the tray) to show boundary interactions?  The time it takes to setup just doesn’t seem to justify the fact that you can see everything with a simple tray you’ve taken borrowed from your school cafeteria.

Anyway, mini-rant over, in using the ripple tank we are to look at how the waves react when a boundary is placed in the water.  Lo and behold, the waves reflect just like we saw with light!  We then try to look at refraction.  This part was very problematic.  The teacher notes say to put a thin piece of plastic in the water to change the water level.  For us, we did see a change in wavelength, however, we were never able to create a scenario in which the direction changed.  I think that side of things needs a little more refining (it has to be the procedure, it couldn’t be your humble blogger).

After a circle meeting to share the behaviors we saw, we worked on the second worksheet which brings in a new way of showing the behavior of waves: wave-front motion maps.  In some sense, just imagine a ray diagram with lines added to show the wave fronts (I’ll try to add pictures at some point, but the server here is way to slow).

One thing that we noticed in discussing this worksheet is that we needed to build the relationship between index of refraction “n,”  the speed of light in a vacuum, and the speed of light in a given medium.  I’m not sure if that jump to light is needed here, rather I think this might be better served taught separately as just mechanical waves (more on this to come in a later blogpost).

From there, we started a lab to show the diffraction of light.  The group set up a flashlight with a screen (open 3-ring binder) casting a shadow on a wall.  They then asked us, if a laser is placed under the flashlight and just touches the edge of the binder, where will the laser dot hit the wall in relation to the edge of the shadow?  We said that it should hit right at the edge of the shadow.  Once they set it up, we saw that the laser hit a few inches away, on the light side of the shadow’s edge.  I think care needs to be taken when setting this up to make sure that the students see that the laser is directly underneath the flashlight, and that the flashlight is small.  Several “students” (myself included) think that big of a shift seems unlikely to be due just to diffraction.

From there we move into a new behavior of light, that being diffraction.  By placing two boundaries in the water, we could look at what happens as the wave front passes between a small opening.  We were encouraged by the “teachers” to look at how the size of the opening effected the resulting diffraction pattern as well as how adjusting the frequency of the source on the diffraction pattern.

We finished up by working on worksheet 3: Diffraction to apply some of the concepts just developed.  One thing I really like was at the end of the worksheet.  The students are asked to predict what should happen if there are two openings.  After a good discussion, I left thinking, whatever the kids think here, as long as they can defend it using the wave model, is ok.  We now have a natural tie-in to a behavior of light not yet studied.  I’m ok leaving the question without a definite solution, as they’ll get it in the very next step of the unit, but that’s for the next blogpost.

{The Wave Model, brought to you by FIU, CHEPRO and the National Science Foundation}

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2 responses to “Light Reflection, Refraction, & Diffraction

  1. Thanks for sharing. Think I’ll try that diffraction demo with the laser and notebook.
    Does the workshop attempt to model amplitude as well?

  2. Thanks for the article. I am also a physics teacher.

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