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Re: [Phys-l] index of refraction



"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'm curious how they did this via inquiry. If you give them the equation,
it's not inquiry. Without the equation, they'd need several different
wavelengths of laser and several different slit separations. "Years ago" I
would guess that several wavelengths of laser would be hard (or expensive)
to find. How'd they do it?

M. Horton

----- Original Message -----
From: "Paul Lulai" <plulai@stanthony.k12.mn.us>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Tuesday, May 12, 2009 12:45 PM
Subject: Re: [Phys-l] index of refraction


Hi.

A few points.

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.

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. 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. 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).

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.

I never spoke of using human error as a source of uncertainty. I wouldn't
accept it any more than you would. I do things very similar to what you
suggest:
" It is much better to design experiments that have no pre-known answer,
and show them how to 1) estimate a statistical uncertainty value, and 2)
look critically at the experimental setup and try to figure out what, if
any, systematic error might be present due to the experimental design."
However, we do not estimate a statistical uncertainty value, we do your item
2). In the beginning of the year they do this as a reflection at the end of
the lab. By the third lab, they are to think about these things before they
conduct the lab. They should then plan their lab in a manner that reduces
as many systematic errors as possible.

I understand the point that these values (index of refraction) can be
calculated with one measurement of incidence and refraction. However, a
plot of sin(theta-i) vs sin(theta-r) yields a nice y=mx plot. This is, in
my opinion, a nice way to get the kids to understand some of the ways in
which mathematical models are applied and created. The students must justify
if the y-intercept should be zero or not. They can then determine what the
slopes might represent. This becomes more obvious if different groups are
conducting the lab with different solutions. If I have each group conduct
the lab with a different solution, they *should* all be able to get plots of
similar quality. The linear best-fit lines (Excel best-fit lines as opposed
to true regression lines) with R^2 values should have R^2 values in the same
general neighborhood. If so, then we can compare slopes etc... Many of the
labs that one requires students to plot data and analyze values could be
done with one data point or two. That doesn't make one-data point labs
preferable. Just quicker.


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

p.s.
A nice way to conduct the resonance lab without a 0.4 multiplier to correct
for end effects: Find the first & second points of constructive interference
(n=1, n=3). The distance btn these two points is 1/2 wavelength. When the
kids ask why they can't use 4*L(1), you have a nice discussion on your hands
and the opportunity to have some students conduct the lab each way. Then
compare results.

A nice momentum lab that involves inquiry from the University of MN (with
apologies for the lack of a tinyurl):
<
http://groups.physics.umn.edu/physed/Research/Lab%20Manuals/1101/1101%20Lab%206%20-%20Momentum.pdf>



Paul Lulai (where I'm attempting to listen and stand my ground while not
stepping on toes).
Physics Instructor, Science Olympiad Coach,
.: Medtronic - St Anthony RoboHuskie Team 2574:. Faculty Advisor

Saint Anthony Village Senior High School, ISD 282
3303 33rd Avenue N.E.
Saint Anthony Village, MN 55418
(w) 612-706-1144
(fax) 612-706-1140

http://www.robohuskie.com
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Education comes from within; you get it by struggle and effort and thought.
Napoleon Hill
One had to be a Newton to notice that the moon is falling, when everyone
sees that it doesn't fall. Paul Valéry
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