The following FAQ contains frequently asked questions and answers about the "Measuring the Sugar Content on a Liquid with a Laser Pointer" (http://www.sciencebuddies.org/science-fair-projects/project_ideas/Phys_p028.shtml) project. If you are having trouble with the procedure, you may find assistance in the answers below.Q: How close to the wall should my prism and laser pointer be set up? A:
You should set up the laser pointer and prism two to three feet from the wall. This will allow you to easily measure the distance between points on the table and points on the wall.Q: When I shine the laser pointer through the prism, I see a red line, not a red dot, on the wall. How can I fix this? A:
You may be shining the laser through the corner of the prism, or the laser pointer may not be perpendicular to the wall. First, check that the laser pointer is perpendicular to the wall. If you still have this problem once the laser pointer is perpendicular to the wall, then the issue is likely the prism's orientation. Rotate the prism until you see a red dot on the wall, instead of a red line. Q: I cannot see the refracted beam in the prism. Where is it? A:
The beam is in between points d and e. When the prism is filled with plain water, the beam can be difficult to see. The refracted beam will become easier to see as you add more sugar to the solution. Try looking down on the prism from above; sometimes this makes seeing the beam easier. You may also find it helpful to line a straight edge (like a ruler) up with points d and e on top of the prism so that you know where to look for the beam. You can also try adding a pinch of non-dairy creamer to the plain water to make the beam easier to see.Q: How do I calculate the arctangent of x/L using http://www.ecalc.com? A:
First, find out what units the calculator is using to measure angles. To do so, click the blue "Menu" button at the upper right corner of the calculator's
virtual keypad. The "Angle" row shows "DEG" for degrees, "RAD" for radians, and "GRAD" for gradians. Click on the unit that you measured your angles in, and
then click "done," the gray button at the lower right corner of the calculator's virtual display. Then click the "atan" button near the upper left corner of the virtual keypad, type in x, click the "÷" button, type in L, click the ")" button, and then click the "=" sign. The answer you will get from this is the angle of minimum deviation, θmd
, in whichever units you selected.Q: I cannot get my laser to consistently hit the same point on the wall when it goes through the empty prism. How do I fix this? A:
Your laser is probably not fastened tightly enough to whatever it is sitting on. Rotate the laser so the on/off button is facing up, and then tape the laser pointer securely in place with multiple pieces of masking tape. Make sure it cannot turn very much in any direction, and be careful not to move the laser pointer when you turn it on and off. In addition, if you have attached your laser pointer to an object (like a piece of cardboard) to raise it from the table's surface, use masking tape to secure that object to the table.Q: The refracted beam (seen at point a) does not move when I try different sugar levels. What should I do?A:
The point a will not move very much from solution to solution; the shifts will be slight. Make sure that your paper is securely fastened to the wall, that you are being very careful in your measurements and that you are not moving the laser. Point a may only move 1/2 a centimeter from one solution to the next so it is okay if you are not seeing large differences.Q: I am not getting 1.334 as my index of refraction for the water. Am I doing something wrong?A:
It is perfectly fine if you do not get exactly
1.334 as your index of refraction. As long as you are consistent in your measurements, you can compare the indices of refraction of the different sugar samples.
Your measured index of refraction may be off due to inaccuracies in data collection or problems in your calculations. If you have a value around 1.334, in the 1.32 to 1.34 range, your error is most likely from your measurements, but it is good to double check your calculations.
If your index of refraction is very far from 1.334 you probably have a problem in your calculations. You should redo your calculations (paying attention to order of operations and checking for consistent units). Also check that the experiment is set up correctly and that you are measuring the correct distances
If you have other questions about the procedure or need assistance troubleshooting your "Measuring the Sugar Content on a Liquid with a Laser Pointer" project, please post your question in the forum for this kit at Ask an Expert: http://www.sciencebuddies.org/science-fair-projects/phpBB3/viewforum.php?f=70
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