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Re: [Phys-l] index of refraction

Will they hold the book value n of vinegar as "holy writ" after we've shown that the book value for g is incorrect for our location?

I was ready to tell you that students have already googled dvd & cd 'groove' spacings. You're nylon idea is nice. I attempt to make many labs problem solving labs. Many of the U of MN general physics labs are conducted in this way. I like the approach. I am not sure how I would incorporate this into the fabric or painted slides. The fabric is interesting. I've never enjoyed getting a set of slides or construction paper with a pair of evenly spaced slits. The precious clock intervenes and I am forced to go with what is convenient.

One experiment that I like that connects macroscopic measurements
with microscopic ones uses the nylon cloth as a starting point. They
measure the thread spacing of the cloth by counting the threads in a
distance large compared to the spot size of a laser beam. Then they
use that spacing to measure the wavelength of the laser, and then use
the wavelength thus measured to measure the groove spacing on a CD or
DVD. It provides a direct connection between something that they can
measure directly by eye and something that they cannot see at all.

Doesn't finding the wavelength of the laser go against your earlier comments?

I understand your point regarding percent difference. I do believe it is valuable in some instances. I understand your contention that students will attempt to get the book/internet value. However, I believe there are valuable instances where I can use a % difference calculation to get students to question not only their own procedural issues, but the conceptual underpinnings of the measurement. Why would the book value for n-bleach be different than ours? Under what circumstances is this acceptable?

All of this said, I feel like I am arguing two points under one mask.
1- Percent Difference - I don't hold to this arguement with great vigor (or frequency). I occasionally use this to compare values from different groups. Occasionally for things like coefficient of friction values. I prefer to collect data and plot it out. Lod are ok having a different u for their wood/concrete interaction that the book. That is not 'holy writ' in their minds. They understand different types and finishes of wood and concrete. See if we get a y=mx+b style plot or something else. What can we get from the mathematical model? What do the m and b values represent? How about area under the curve?

2- Lab types. As Hugh himself states (or implies with his dvd - laser - fiber lab), it is possible to take a lab that has a verification flavor, but turn it on its side a bit and get a decent lab. Rick makes a similar claim with pendulums. When we are done with Rick's pendulum lab, we have a mathematical model that is the same (hopefully) as the accepted model for the period of a pendulum. That isn't a verification lab. I wouldn't claim using Young's Double slit lab to find the slit spacing of a dvd, cd, nylon, razor blade, or other item to necessarily be a verification lab.



Paul Lulai (apologizes for a terribly inefficient use of text)
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________________________________

From: phys-l-bounces@carnot.physics.buffalo.edu on behalf of Hugh Haskell
Sent: Tue 5/12/2009 10:16 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] index of refraction



At 14:45 -0500 05/12/2009, Paul Lulai wrote:

First, we do not do percent error calculations, but % difference
calculations. I believe Hugh's comments are more directly
applicable to percent error calculations. I might be wrong.

If the % difference is between your measured value and a "book" value
(same as "accepted" value), the I see % difference and % error as the
same thing, and the students will treat it as such.

My students are to determine why the two values might be different.
I specifically state that we should not hold textbook values in too
high of a regard.

Never mind. They will hold the book value as "holy writ," and do
whatever they can to get their results to agree with the book value.

They must state what might cause these differences, how their
laboratory procedure might have caused these differences, and what
could be done to change the procedure to limit the error involved.
I do not give them the 'accepted' values of these substances.

OK. But if you give them a "book value" they will treat it as such.
And if you give them something that has an "accepted value" to work
with, they will, as you have clearly pointed out, find it on the
internet or elsewhere, is that is what they will be looking for in
the lab.

The solution to that is to not give them standard constants to
evaluate in the lab.

Kids today will google or Wikipedia the value. If I don't ask them
to do a % diff, some (not all) will attempt to fudge data to match
the internet value. I've done labs along these lines to determine
an unknown item in the past (maybe use n or C to determine what
solution or metal sample you have been handed).

If there is nothing for them to compare their result to, then they
can't fudge their data to meet any predetermined answer.

None of my labs are cookbook recipe labs. I rarely conduct
verification labs. However, students are becoming very adept at
finding some of the information that I ask them to determine. Years
ago I would ask them to use a double slit to find the wavelength of
a laser. Then kids could more easily find the wavelength. I
switched to find the slit separation. Now kids google the little
CAS image on the Cornell slides I've got and they know the openings
on the slides. I now have to find a new version of Young's Double
slit. Don't know what that will be, but it has never been a
verification lab.

CDs and DVDs make good slit sources, as do woven materials The
diffraction patterns created by a nylon woven cloth (get one with as
little stretch as possible, and the smooth fibers of nylon or other
synthetics make much more uniform patterns) are very interesting and
should pique the curiosity of at least a few of the students. Or use
closely spaced razor blades to make a single slit with a different
spacing for each group (stretching a nylon thread between the edges
of the razor blade slit will turn it into double slit) , or scratch
slits on a painted microscope slide (the biologists might have some
devices that will allow you to make well-defined scratches. All you
have to be is a little cleverer than the students are and you should
be able to provide them with material to measure that no one has
knowledge of the results to be obtained.

One experiment that I like that connects macroscopic measurements
with microscopic ones uses the nylon cloth as a starting point. They
measure the thread spacing of the cloth by counting the threads in a
distance large compared to the spot size of a laser beam. Then they
use that spacing to measure the wavelength of the laser, and then use
the wavelength thus measured to measure the groove spacing on a CD or
DVD. It provides a direct connection between something that they can
measure directly by eye and something that they cannot see at all.

One of the things I like to emphasize in introductory physics classes
is the question "How do we know that?" So during the course of the
year they see how we measure the scale of the solar system, using
Kepler's laws and the parallax of asteroids to determine the distance
to the sun (or the variation in the eclipse time of Saturn's
moons--once we have a terrestrial measurement of the speed of light),
then extending it to nearby stars using parallax, more distant stars
using Cepheid variables and color-brightness scales, and finally to
distant galaxies using red shift. We also discuss how to measure the
size of atoms and other quantum particles so they get an idea of how
we figure out what goes on in those areas where we cannot see or
measure directly.

If there is one thing that I want my introductory students to take
away from my class, it is the knowledge that the measuring of the
very large and very small starts with very mundane measurements of
ordinary things here on earth and uses those measurements as a
jumping off point for all the rest. So they know that there is a
connection between those things and our everyday reality, even if the
connection is complex, it is a very human effort, and if they are
interested, they can learn how to do it and make their own
measurements, hopefully of things no one has measured before.

Too often students come away from science classes with the idea that
this stuff is just magic, and no one really knows such things as the
scale of the universe, or the size of atoms--it's all just made up
stuff to confuse students. So it's important for them to see how we
really can find some very interesting things out just using some
everyday stuff and some careful thinking.

So they do need to measure some fundamental things, like g, and stuff
like that, but I have them do it so early in the course that they
don't really know what it is they are measuring until later when we
put that information to good use. Meanwhile, they have had a
smattering of experience with error analysis, and they understand
that the values they got in the lab will need to be corrected later.

I never spoke of using human error as a source of uncertainty. I
wouldn't accept it any more than you would.

I didn't accuse you of that. I just tossed it out as a rule I enforce
about error analysis. Too many of their prior teachers have used that
to explain away crude results that don't agree very well with theory,
so I do my best to squelch that tendency immediately.

I am not sure what Hugh means by an accepted value of 0.

Any physical value that must be zero for certain situations or
objects. For example, a collision in which the initial total momentum
is zero. It must also be zero after the collision, but on an air
track, it seldom is exactly zero due to factors that are very
difficult to control. If you are asking them to calculate the %
difference in before and after momentum, they will of necessity get
an enormous value for that percentage. There are some physical
properties of materials that have a theoretical value of exactly
zero. Most of them are in advanced topics so first-year students are
unlikely to encounter them but they exist. Measuring them involves a
careful determination of the error bars of the experiment to
determine if the theoretical value is included within the error bars.
If so, there is no discrepancy with experiment and theory, so the
experimenter will hope that they have narrowed the uncertainty in the
experimental value. On the other hand, if the error bars do not
include the theoretical value of zero, then it is important for the
experimenter to be very sure that there is no remaining systematic
error that can bring the value back to within the theoretical
expectation, but if none is found, then something very important has
been discovered--namely that there is something wrong with the
theory. This is, of course, true for any theoretical value, not just
ones that are presumed to be zero, but the zero values have a special
nature in that one cannot identify the difference between experiment
and theory with a percentage value.

A former student of mine, when she got to grad school, got into a
project in which the goal was to measure the quadrupole moment of the
electron, which is, I believe, supposed to be zero, indicating that
the electron is a true point particle. Clearly, if the results came
out to be unambiguously non-zero, no matter how small, it would
change much of the current thinking about the nature of electrons. As
to her results, alas, love intervened and she quit grad school to get
married before the project got much off the ground. But I think that
if anyone had found that the value is non-zero, we'd have all heard
about it by now.

Of course, we can't expect students to discover everything. There
isn't enough time in a one year course to cover the extent of 500 or
so years of physics by doing everything in the lab. But they need to
understand that everything they will have given to them over the
course of the year, has its origin in exactly the sort of lab
exercise that they have spend their lab periods doing over the course
of the year. So we hope to give them at least a hint of what real
science does, even if we can't take the time to recreate the entire
process.

Hugh

--
Hugh Haskell
mailto:hugh@ieer.org
mailto:hhaskell@mindspring,.com

So-called "global warming" is just a secret ploy by wacko
tree-huggers to make America energy independent, clean our air and
water, improve the fuel efficiency of our vehicles, kick-start
21st-century industries, and make our cities safer. Don't let them
get away with it!!

Chip Giller, Founder, Grist.org
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