Another good reason never to answer questions on tests -- as if you needed one

At the AP physics reading, I graded problem B3, about single slit diffraction.  The rubric will be released in September -- suffice it to say that the student who did everything right but incorrectly treated the plates as a double slit could get all but two points.

Once every few hundred exams I would see a comment to the effect of "If this was supposed to be Young's experiment, it is lacking an extra slit to allow constructive and destructive interference between the waves."  Okay, these students missed the point -- even single slits show a diffraction pattern, and it's a common misconception to think otherwise.  Certainly more than just those who expressed their bafflement in writing shared this lack of understanding.

The key to success on a physics test, and, I believe, the key to thinking like a physicist, is how a student reacts to such a conundrum.  "The question doesn't seem to make sense.  What do I do?"  What do we *want* our students to do?

The ideal reaction is to write the issue as the student sees it.  "This sure looks like an interference problem, but how can interference happen with a single slit?"  And then, to attack the problem somehow, someway:  "Well, I'm going to treat this like a double slit anyway and see what happens."  I saw students using just this approach, and doing well.

The poor reaction is for a student to become offended and huffy: "Single slits don't diffract.  What the heck do you want me to do, then?"  "My teacher never taught me to do single slits.  Can you call my school and get him fired?"  "What a dumb question.  We never studied this, just double slits."* 

*Note that these are not verbatim student responses, just general impressions of things students have written.

It's easy enough for me and my physics teaching friends to get haughty about the poor reaction.  Damn fool kids these days, they don't study, they don't listen, etc.*  But never forget that it is our responsibility not just to teach the material, but to prepare our students for a comprehensive exam.  The students who fight through their initial confusion to get substantial partial credit weren't necessarily smarter than the rest, but they likely had been prepared for just this sort of situation.

*Kids in our day studied, listened, respected their elders, and never texted in class.

Consider how your students might react to this question on an in-class test.  Early in the school year, every student in America would want to come to the teacher's desk to say, "shouldn't this be a double slit so there can be interference?"  Our job as physics teachers is to prepare the students to have their ability to ask questions eliminated.  It's a physics test, not a test of how well you can read the teacher's face or coax information out of him. 

I talk to my classes BEFORE our first test of the year, explaining how to handle just this sort of situation.  Still, they will try to ask questions during the first test, and it's likely they will make me very angry.*  The class learns that I mean business -- they have to figure out test questions on their own.

*While I am always a loud guy, the only times I will become truly *angry* at a student might involve blatant disrespect,, laziness combined with arrogance, or ignoring my rules about questions during tests.

I would bet that many of the folks who became upset about the single slit were frustrated because they were used to convincing their teacher to talk them through difficult questions, and that couldn't happen on the AP exam.  You and I can't possibly teach well enough that every student is fully comfortable on every question.  But by testing regularly under authentic conditions, and then debriefing after each test, it is possible to train our students how to react when they're not comfortable.  That's a skill that our graduates will thank us for someday.

GCJ

Going over a test

I know it's happened to you and it's frustrated you.  You give back a test, you discuss one of the more frequently missed questions, hoping for a teachable moment.  But half the class is rooting through the rest of the test, sitting back with a vacant expression, or simply absent mentally.  What to do?

One option, which I've discussed before, is to allow corrections for half credit.  Then there's no need for you to take too much class time to go over the test -- it's the students' job to figure out what they missed, and to convince you they understand now.  A related idea is to announce a "fundamentals quiz" over commonly missed concepts from the test.  Either way, the students are forced to think about the test beyond just "what did I get?"

Of course, test corrections are time- and manpower-intensive. You have to give time in or out of class to get the corrections done, you have to grade them as thoroughly as you would a test.  I only do corrections in my AP class -- I find the general class moves slowly enough that those who missed important points will pick them up soon. 

So how do I go over a test in general physics?  Well, keeping my comments brief and to the point helps.  But the key little trick is to HOLD THE TESTS IN MY HAND while I go over them. 

Here I'm playing with the students' minds.  They desperately want their tests back, but only so they can see the grade.  Once they see that grade, their mind is done for a while, and they don't want to think about physics.  So I use the grade as a carrot.  I dangle the papers with the grades on them right in front of the class.  Not obviously or obnoxiously, of course, but they are never sure when I'm going to shut up and hand out the tests.  And, they're nervous about what they did right or wrong.

So they listen.  And ask questions.  They want to hear what I say, so they can figure out whether they were right or wrong.  The same discussion AFTER I give the tests back would be fruitless.

How do I know this technique works?  Well, I don't for sure.  But I do note that folks occasionally note to their friends whether they did or didn't make the mistakes that I discussed... so they must have paid some attention.

GCJ