Reminders from TA/LA Training on 4/03/2015 for week of 4/06 (Lab 9, Part 2)

1) Recitation: Diatomic Vibrations

Link: Diatomic vibrations

(Break--Pass back graded Lab 8, if you have not already done so...)

2) Lab 9, Part 2: Exploring Light and Lenses

Link: NEXUS Physics Labs, 2013-2014

* Before lab starts, make sure that the previous TA left everything in order: check that all the plastic trays in the back-left cabinet have the needed equipment AND check that all the 90-degree clamps have been removed from the vertical pole clamped to the table.  Also check that all the lamps have been turned off/unplugged AND that all the LED light-boxes are switched off.  Be kind and leave the equipment in order for the section that comes after yours!

a) Safety and equipment instructions:  If students will be taking data this week....  Dangers in this lab include the glass lenses (breakable and sharp when broken) and the HOT light sources (turn off/unplug when not in use)--remind about handling the equipment as you feel necessary.  Additionally, the vertical pole clamped to the table should be left where it is--no one should move it.  I recommend TAs/LAs wear 100% cotton shirts so that you can quickly and easily clean the lenses, when necessary.  Please tell the students not to clean the lenses on their own clothing (as they are rarely wearing a safe fabric).

b) Check that students have completed their measurements for the five lenses, including measuring uncertainties and magnifications (image heights).  If necessary, the first 10 minutes of this lab can be run in a 'dark' room to complete the data collection.

c) The trick in Lab 9, Part 2 is in encouraging students to use graphs to display their data and use graphs w/ error bars to test a variety of fits.  They have lots of options for what to plot.  As with all inductive modeling, they should start with simple plots (one variable vs another) and try for simple fits first (linear, then quadratic, then exponential...).  Their report should include a variety of plots that they have tried, along with a discussion of which mathematical model(s) best fit the data and why they have decided this.  If students do the i vs. o fit (a perfect, negatively sloped line, with R^2 = 1), help them see that this is just the graphical/mathematical form of the fixed L requirement--their experimental set-up!  Some students may do a thin lens equation graph.  Don't stop them, but encourage them to try other fits, too, as experimental uncertainties should result in a parabolic fit (the start of a Taylor expansion--which can fit anything) having a better R^2 value.  Students should also discuss any magnification findings--though coming up with a mathematical model for this is not necessary.

d) Students should find f vs. o and f vs. i to be downwards opening parabolas, with -1/L as the leading coefficient and "1" as the coefficient of the linear term.  Idealizing this model (and ignoring the constant term) they should be able to turn the fit equation from the data into the thin lens equation (which they will learn in class in a few weeks)--do this in individual groups or as a class, following the poster presentations.  Some students may not get this result as their best fit (depending on how carefully they constructed and executed their experimental design).  Still, by having each group present their findings to their peers (in 'poster session' form or in 'talk' form), students who did not get a parabolic result will be able to see that other groups did get this result (and that the leading coefficient for these parabolas is -1/L, regardless of whether the plot is f vs i or f vs. o).

e) Also, please have students close the "experimental design" loop by discussing how the experiment would need to be modified in order to model/investigate a "simple" (in the biological sense) microscope (a la Darwin).  There are seven of these microscopes--one in each equipment bin (back-left cabinet) and one on the "TA" desk.

f) Leaving the lab in order for the next group: Check that all the trays have the needed equipment AND check that all the 90-degree clamps have been removed from the vertical pole clamped to the table.  Also check that all the lamps have been turned off/unplugged AND that all the LED light-boxes are switched off.  The plastic bins (with all needed equipment AND the simple microscope) should be placed back into the back-left cabinet (so that they are out of the way for the recitation for the next set of students).  Make sure that all the lab papers are also sorted and stacked neatly at the back of the room by the printer.  Be kind and leave the room in order for the section that comes after yours!

Good luck!  And please bring your Lab 9 reports to me in 1322 for scanning when your lab section(s) are done.... :-)