Measuring data track spacing on a CD with a laser pointer

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Measuring data track spacing on a CD with a laser pointer

Postby LaserMatt » Wed Mar 28, 2007 5:29 pm

Hi, I found this science project idea on this website and it was a really cool project. The only problem is, now that I've measured all of the angles of diffraction of the laser on a protractor, I don't know how to analyze the data. Do I subtract 90 degrees from all angles, or only some of them? When I get a negative number for the distance calculation, what does that mean? I used the data table given in the experiment, but don't know how to make common sense of the measurements. What is the spacing? Should a DVD have closer spacing than a CD? How do the numbers correlate with reality? And finally, can the data track spacing be converted to minutes of remaining recording time or something useful? I mean, if all these numbers tell me is the spacing of the data tracks, how is that useful in a practical sense? Please help me out, my project is due in a week but I'm going out of town with my parents on Friday. Thanks in advance.
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Postby Craig_Bridge » Thu Mar 29, 2007 6:08 am

I personally haven't dealt with this experiment before. Others have done it with assistance from other experts. Try looking at this dialog and see if anything in it clears up some of your questions: http://www.sciencebuddies.org/mentoring/phpBB2/viewtopic.php?t=2247

Hopefully another expert who has helped with this one will respond but it might not be within your restricted time frame.
-Craig
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Postby Louise » Thu Mar 29, 2007 7:46 am

Craig_Bridge wrote:I personally haven't dealt with this experiment before. Others have done it with assistance from other experts. Try looking at this dialog and see if anything in it clears up some of your questions: http://www.sciencebuddies.org/mentoring/phpBB2/viewtopic.php?t=2247

Hopefully another expert who has helped with this one will respond but it might not be within your restricted time frame.


Here is another thread which deals with this topic (how to work up the data/idenitify the diffraction orders):
http://www.sciencebuddies.org/mentoring/phpBB2/viewtopic.php?t=2360

There are many more, but I think those two will be most useful. You could also do a search on this forum.

The "correct" answer for DVD and CD track spacing is found in the links (or by googling). You should also read about how CDs and DVDs work (again, either google or look in the bibliography section of the preoject) because that will answer your questions about track spacing and "minutes of recording time or something useful". Most of these questions you pose are very general, and you should have found the answers when doing you background reading.

Louise

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Re: Measuring data track spacing on a CD with a laser pointe

Postby OneBriiguy » Thu Mar 29, 2007 7:58 am

LaserMatt wrote:Hi, I found this science project idea on this website and it was a really cool project. The only problem is, now that I've measured all of the angles of diffraction of the laser on a protractor, I don't know how to analyze the data. Do I subtract 90 degrees from all angles, or only some of them? When I get a negative number for the distance calculation, what does that mean? I used the data table given in the experiment, but don't know how to make common sense of the measurements. What is the spacing? Should a DVD have closer spacing than a CD? How do the numbers correlate with reality? And finally, can the data track spacing be converted to minutes of remaining recording time or something useful? I mean, if all these numbers tell me is the spacing of the data tracks, how is that useful in a practical sense? Please help me out, my project is due in a week but I'm going out of town with my parents on Friday. Thanks in advance.


Hi, LaserMatt!

I'll take a shot at some of your questions.

The angle measurements should all be taken with respect to the vertical axis. In the picture example shown on the project page (See https://www.sciencebuddies.org/mentorin ... ?from=Home), the 90 degree mark on the protractor is supposed to be zero. If you are using the mark on the protractor as the measured value, then you do, indeed, subtract 90 from that value to obtain the diffracted angle. In the example, m = -2 crosses the protractor at 56 degrees, so the angle for m = -2 is (56 - 90) or -34 degrees. Another way to measure this is to count the number of degrees from 90. Anything to the right of vertical is negative, and anything to the left is positive.

I haven't worked through the math myself, but my guess is that a negative value for d is wrong.

The spacing is defined in the project article. I don't know what DVD and CD spacings are relative to one another. The reality of the spacings should become apparent when you read the project description and come to better understand what the spacing is.

I also believe that once you understand what spacing is, you will be able to answer the rest of your questions for yourself.

Best wishes!
Brian Castelli (OneBriiguy)
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