Questions about rotating prism in Measuring Sugar Content of a Liquid with a Laser Pointer

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ChemTeacher44
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Questions about rotating prism in Measuring Sugar Content of a Liquid with a Laser Pointer

Post by ChemTeacher44 »

Hello and thank you in advance.
I'm a teacher guiding a student with the project "Measuring Sugar Content of a Liquid with a Laser Pointer"
https://www.sciencebuddies.org/science- ... er-pointer
and I have some specific questions about the experimental procedure.

1. In step 3 of the section "Standard Sugar Solutions for Comparison", the procedure states to "repeat your measurements ... for a total of 5 replicates". What exactly is the difference between the 5 trials? The solution is not being changed, nor the position of the laser. How will the measured spot be any different?
Is it the rotation of the prism to make the in-prism refracted beam (blue in diagram) parallel to the "unused" face (so as to truly get the angle of minimum deviation)? Thus the 5 trials is to average over the source of error of not getting the rotation/parallel exactly right? Is this understanding correct?

2. To be clear, for each new sugar solution (10%, 20%, 30%), we must rotate the prism to a different angle to make the new inner-prism beam parallel to the opposing face. Is this correct? As above, the instructions are not clear as to what will be different for different iterations of the experiment.

Thank you for your advice!
bfinio
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Re: Questions about rotating prism in Measuring Sugar Content of a Liquid with a Laser Pointer

Post by bfinio »

Hi,

You are correct on both counts. Since the procedure involves a bunch of manual adjustments and measurements (rotating the prism to get the beam parallel and then marking all the points shown in Figure 9), there will be some error with each measurement, which is why we recommend multiple trials. For each new sugar solution you will expect a different angle of refraction and thus need to rotate the prism to a different angle, but generally we try not to "give away" too much in the instructions for the student, which is why we don't say that explicitly. Hope that helps!

Ben
bfinio
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Re: Questions about rotating prism in Measuring Sugar Content of a Liquid with a Laser Pointer

Post by bfinio »

* just to clarify in case this was not clear in my original response - I believe you should start over with a new piece of paper to mark the points in Figure 9 for each measurement. Otherwise as you said, you'd just be re-measuring the same points five times. I would start the entire measurement process over in order to average out the error, and will add a note clarifying that to the procedure.
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