Re: [Phys-l] index of refraction

[Hugh Haskell <hhaskell@mindspring.com>]

At 12:49 -0500 05/11/2009, Paul Lulai wrote:
> That would make it a fairly productive lab.  Additionally, they 
> could calculate % difference btn their values and the 'accepted' 
> values. 
I would argue that any lab in which the goal is to measure the % 
difference between their value and the "accepted" value teaches the 
students nothing of value, and in most cases does just the opposite. 
First, it teaches the students that experiments are not to find out 
anything new, but to verify what we already know, and second it leads 
to what we used to call in the Navy, "gundecking" the results--that 
is, making the results give the "right" answer. Furthermore, the 
"accepted" value is the accumulated best experimental results, and 
not in any sense "correct." so calculating the % difference between 
the two is pretty meaningless, and particularly confusing when the 
"accepted value is zero, since any % difference from zero is 
automatically infinite.

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.

When I have something that I want them to do that does involve 
comparing their results to a "known" value or idea, I try to design 
the experiment so that the expected result is zero, and then tell 
them to estimate their "error bars" and see if that range of values 
includes the "known" value. If so, they should be disappointed, since 
they didn't discover anything new. If not, then they need to figure 
out what they did that might have led them to get a result that 
doesn't agree with the consensus, and if they can't find something 
that might have gone wrong, then they need to offer a possible 
hypothesis for testing that "explains" the difference between their 
result and the "accepted" result.

Furthermore, using the phrase "human error," to explain any anomalous 
result, earns them an automatic failing grade for that lab. They can 
say "I figured out that I measured A using an incorrect method," 
(provided they describe the incorrect method, and how they could have 
done it right) but they cannot say, "I must have measured A 
incorrectly."

The sooner students learn that science experiments are not for the 
purpose of verifying what we already know, but for trying to discover 
something new, or at least to confirm a previously unconfirmed 
hypothesis, the better off they will be.

Hugh

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Hugh Haskell
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mailto:hhaskell@mindspring,.com

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