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## 25 March 2010

### A different use of a clicker quiz -- snell's law

Springtime for seniors brings a competition as to who can do the least amount of work.  That's certainly not true for all seniors, but if you listen to any high school faculty this time of year, you'd think that this year's crop of 17 year olds were the passive-aggressive scourge of Satan.  In a previous note, I explain my use of the "exemption" as one prong in my defense against the senior slide.

The gist of the exemption:  Students who put in particularly strong effort on out-of-class assignments earn the right to skip a future assignment of their choice.  The first exemption of the year in general physics is awarded to the student with the highest homework average from the previous trimester.  From here on, exemptions will be given for things like maintaining an A homework average over the course of a full week, writing a perfect quiz when that quiz is based on a homework assignment, or a perfect fundamentals quiz.

The homework problem for today involved refraction in a triangular block of glass.  I've introduced Snell's Law already, and we've done several basic problems with simple geometry.  Today's problem was more complicated, because the normal was NOT straight up and down the page.  Take a look at the diagram to the right, which is slightly editied from a problem in the Glencoe text.  The homework problem did not label the 49 degree angle; instead, it gave θi as 45 degrees, and asked to find angles A and B, and θexit

I warned everyone ahead of time that angles A and B are NOT 60 degree angles.  That doesn't prevent half the class from making that assumption anyway, but it sets up for success those students who pay attention.

For today's quiz, I gave the diagram shown, with the 49 degree angle.  I asked the six questions at the end of this post, to be answered on the classroom response system (the clickers).

Now, I am careful NOT to use the clickers for quiz purposes through most of the year.  At first, I absolutely do not want students seeing how their peers did.  I don't want boasting about good scores, I don't want the sour-grapes rationalization that inevitably follows when someone gets a 25%.  Over the course of the year, 1/4 on a multiple choice quiz is a drop in the bucket.  My guys can deal with that.  What they CAN'T deal with at first is the idea that the average score on the quiz might only be 50%.

In the spring, though, I use the clickers because I WANT score distributions to be public.  Everyone needs to see that yes, people are getting 6 correct answers on this quiz.  The clickers' instant feedback allows me to pinpoint the folks who missed a straightforward question, and make sure they know why they got it wrong.  And most importantly... when the quiz is over, I can IMMEDIATELY and publicly award an exemption to everyone who got a perfect score.

GCJ

1. What is the angle of incidence at the left edge, labeled θi in the diagram?

2. What is the angle of refraction at the left edge, labeled r in the diagram?

3. What is angle A?

4. What is angle B?

5. What is the angle of incidence at the right edge?

6. What is θexit?