finding data track spacing on cds/dvds using a laser pointer

[deleted-172209]

I was doing this experiment but my data seamed to be off and I was wondering if you could check it for me.

Thank you
Ryan

[deleted-71603]

Hello, and welcome to Science Buddies!

This may be an easy fix. When dealing with the SIN function in Excel, it is expecting an input of radians, not degrees, for your angle. Why does Excel do this? Radians are the standard unit of angular measure. Degrees are a different form (and typically easier for people to understand) to express the same value, but you can't use degrees in formulas. You must use the radian value. Just as you can't use 95% in formulas...you must use 0.95, instead.

To convert degrees to radians, simply multiply your degrees by pi/180.

So, in Excel, if your degrees value is 80, your function to convert to radians will be: "=80*pi()/180"
There is a RADIANS function in Excel, as well. Your SIN formula would be ="SIN(RADIANS(80)"
Both formulas should yield the same answer.

Try this in all of your calculations and see if the data looks better. Feel free to write back if you have any more questions. Good luck!

[deleted-172209]

I changed my data and I was wondering if you could check it for me.

Ryan

[deleted-71603]

It's a little difficult to follow the flow of your spreadsheet. I think your end results are I4-N9, but I'm not sure. Your calculations seem correct, but there is a lot of variance in your results. What do YOU think about your data? Is this large variance ok?

It seems you changed the angle of your incident beam for every trial. The procedures say it is ok to do this. Unless you are doing something else different between trials, changing the incident beam could be the cause of your large variance, which could make it difficult to draw conclusions. Perhaps do the experiment again, and keep the incident beam the same across all 5 trials. See if the results are more consistent.

If this happens, hey, you had a happy accident! You could have discovered that the angle of the incident beam DOES matter. If this is the case, you will certainly want to include this in your presentation; and let us know so that we can update the procedures.

Let me know your progress! Good luck.

[deleted-172209]

If I had the same angle across all 5 trials, wouldn't the trial results be the same?

Ryan

[deleted-71603]

It is very possible that you will get the same results, but not necessarily. Small fluctuations in how you conduct your experiment can produce different results. What are some things in your experiment that could cause differences in measurements?

- The angle of your incident beam? This is currently in question.
- The placement of your protractor on your CD / DVD. Is it level every time?
- Reading the angle off of the protractor. If you read the protractor directly in front, off to the right, off to the left...is it the same result every time? For example, reading the protractor directly on may show 90 degrees. Take one step to the left, and it may read 91 degrees. Take one step to the right, and it may read 89 degrees. Should you attempt to read the protractor from the same angle every time?
- Are you using the same CD / DVD every time or a different CD / DVD?
- Anything else?

Repeating an experiment ensures your results from one trial isn't an accident. Also, whenever a human is involved in an experiment, small variances will occur for some of the reasons listed above. In more advanced testing techniques, formulas are used to calculate the number of trials required to have high confidence that the results of your test are representative of what happens in reality. For your test, conducting five trials will be sufficient.

A good experiment will control as many variables as possible. However, not all variables can be controlled. By doing multiple trials and then averaging the results, you attempt to account for these small variations. You can read more here:
https://www.sciencebuddies.org/science- ... l#overview

I hope this helps.

[deleted-172209]

I was wondering if you could help me figure out why the numbers that are highlighted in orange are so far off from the rest of the data.

Thank you,
Ryan

[deleted-172209]

I also could not understand what the final result would be.

Ryan

[deleted-172209]

I did the experiment again with just one indecent angle.

I was wondering what d stands for.

Ryan

[deleted-71603]

Let's concentrate on your Sheet2, where you kept your incident beam angle the same across all trials. From your data on Sheet1, it seems the incident angle may, in fact, make a difference. We'll submit the procedures for review.

On Sheet 2, everything looks good. D is the data track spacing...how far apart data is stored on the CD / DVD. Your average spacing numbers are different for CDs and DVDs. DVDs can hold more data than CDs. Do you think there is a correlation here?

[deleted-172209]

Would the final result be the average of all the Ds ?

Ryan

[deleted-71603]

Yes. Notice your average track spacing is larger on a CD, which holds less data. Does this make sense? Why?

[deleted-172209]

Also, if I did all of the measurements at 60 degrees, would the incident angle be 30 or -30 degrees?

Ryan

[deleted-172209]

Yes, I makes sense because if the spacing is larger, and the cd and the dvd are the same size, then the track on the cd would be shorter.

Ryan

[deleted-172209]

Thank you for help.

I am still trying to understand what the individual d values actual mean. For example, for the CD measures, my average d value for m=1 is -1.409, and my average d value for m=2 is 3.0679. So, what is -1.409? Is it a distance in nm from the focal point of the laser? And since it a negative number, does that mean it is a distance in the opposite direction from the 3.0679 measure? This is what is confusing to me. If what I describe above is actually the correct meaning of the values, then shouldn't I be taking the average of the absolute values of the d values? Also, if I did all of the measurements at 60 degrees, would the incident angle be 30 or -30 degrees?
Ryan

[deleted-71603]

CDs and DVDs have a standard track spacing. They are referred to as "pits," as described here:
https://www.sciencebuddies.org/science- ... background

What are the standard sizes of pits for CDs and DVDs? You can read more information here:
http://en.wikipedia.org/wiki/Dvd

What you are attempting to do in this experiment is to measure this pits. Your end results should be similar to the numbers stated in the wikipedia article. (Measurements given in micrometers). Notice in the article that the pit size is about half on a DVD compare to the size on a CD.

You are correct in that you should average the absolute value of all of your results. When you do that, your calculations are spot on for CDs, but not for DVDs. Perhaps try to take the measurements again for your DVD to see if there was an error in your measurement. You only have two diffracted beams. Try different incident beam angles to see if you can get more diffracted beams, which will give more accurate results.

Here is a good thread on your same topic:
https://www.sciencebuddies.org/science- ... =29&t=3955

I hope this helps.