Science Buddies: "Ask an Expert"

I have this project where I have to measure the distance between the information tracks on a CD using diffracted light from a laser. I have measured all the needed values and calculated the distance between the tracks. I used two different lasers, a CD and DVD. Now the last calculation I need to do is to find the number of tracks on the CD per milimeter. I have, for example that the distance is 907 nanometers and I can't figure out how to calculate the number of tracks per milimeter according to this distance. Help would be appreciated

TheBosnianDragon - To keep units and decimal points simplified, lets use a millimeter as our unit of length. 907 nanometers is expressed as 0.000907 millimeters. If you divide 1 mm by 0.000907 you get 1102.5, or that many tracks per millimeter.

Rick Marz

Rick Marz - Thank you so much for that! Really helps me with completing this project. I don't know how I haven't thought of that myself
Another thing I wanted to ask: I've done measurements on a CD and DVD using two lasers, one was a red laser of wavelength 650 nanometers and another green laser of wavelength 532 nanometers. When I measured measured the distance between diffraction spots of the lasers from the CD and DVD I noticed that there is a fairly large difference in the final result for the track distancing when using the different lasers. What this means is that I got the result for the CD track distancing to be 907 nanometers with the red laser and only 887 nanometers with the green laser. I'm wondering if this can be attributed to the different wavelength of light or is it maybe due to some miscalculation I made or possibly due to my inconsistent measuring of the height of the laser and the angle the CD was suspended at?

TheBosnianDragon - The wavelength of the laser, green or red, shouldn't introduce any difference in measuring track spacing. The difference you noted, 907 versus 887 nanometers is only a 2% difference. My guess is that your setup and measurement could have created that amount of error. You might try another attempt at measuring angles, perhaps you'll see a lower variance.

Rick Marz

Rick Marz - Thank you for all your help. I might try to measure it one more time or maybe I will just state that as an error during measurements.