Ask an Expert: problems with measurements

I finished the experiment and have a question about results. My index numbers for each of the sugar preparations get smaller as the percentage goes up, and all of them and the angles are smaller than the one with plain water. My question is: shouldn't I expect the angles to increase as the percentage of sugar increases since it should slow the light down and make it "bend" more?

I agree that something is wrong with your proceedure or results. What equipment are you using to measure the angles? Did you make a 60 degree hollow prism from glass slides or are you using a "D" shaped refraction tray?

If you are using a 60 degree hollow prism, did you re-allign the light path through the prism to be parallel to the third side (the one the light doesn't enter or exit from for each concentration? If you are using a "D" shaped refraction tray, are you measuring the exit angle as the deviation from the straight through point? If not, then 90 degrees - your measured angle might be the exit angle.

So a follow up question then is how to rotate the prism. Should it have a fixed point in the center, or at the far point (tip) to rotate so the untouched side is parallel to the beam? Thanks again for all your help!

Hello miles604,

I'm coming in late to this discussion. You seem to have discussed eariler, but I find no additional posts under your name. I will assume that you are following the experiment at https://www.sciencebuddies.org/science-fair-projects/project_ideas/Phys_p028.shtml#summary.

While the fine points of how the prism is aligned are critical for getting good measurements of the refraction angle, I am most concerned that you are not seeing an increasing total angle of refraction as you increase the amount of sugar. The increasing angle vs. sugar content should be seen even with a wide range of alignments.

I suggest that you set up an alignment like that shown in Figure 7 of the Procedure section and leave it in place. Then try putting clean water and then a strong sugar solution in the prism. If you don't see a larger refraction angle and a corresponding larger x as defined by Figure 7 when you change from water to sugar solution, then I think you must not be using plain sugar (sucrose). It's hard to imagine any other explanation.

Once you have verified that sugar solution bends the light more, you can work on refining the setup for accurate measurements.

Good luck, WW