Science Buddies: "Ask an Expert"

[Science Buddies note: Posting this on behalf of Ron.]

Project: https://www.sciencebuddies.org/science- ... p011.shtml

My daughter and I have done this experiment and we have two questions:

1. We were able to duplicate the diagram shown in Step2 “making the measurements” exactly as shown and were able to get measurements for the angles at m = +1, m = 0, m = -1 and m = -2. The table in step 3 for the calculations shows m =1, m = 2, m = -1 and m = -2. We were never able to record an angle for m = +2. Is this normal?
2. Using the recoded values we were able to calculate d of m = 1, m =-1 and m = -2 which all came very close to 1500nm for the CD while for the DVD the values were 724, 794 and 1418 nm respectively. It seems odd that the CD values were also so close to the standard 1600 that is typical of CDs while two of the values for the DVD were very close to the 740 nm that is typical of DVDs. Any suggestions why m = -2 was so far off at 1418 nm? We repeated the experiment several times and recorded very similar results


Please advise.

Best regards

Ron Jagmohan

Hi!

I reviewed the experiment, and hopefully can help with some of your questions.

With regards to not getting data for m=2, I can't say if this is normal, but in looking at the write-up, I see they didn't get a reading for m=2, either. Depending on your incident angle, I would think that the angle of the diffracted ray for m=2 is large enough that it is too weak to see.

Based on your data for the DVD, I'm wondering if maybe the value of 1418 nm might actually be for m =-1. Is there any chance, that due to the incident angle, that you actually measured an angle for m=-1? (I'm making this guess by looking at the equations.) It's also possible that there is a diffracted beam between 3 data tracks (instead of 2), which would cause the distance to be twice what you expect. Things to try would be to rotate the DVD so that different data is under the pointer, trying different DVDs, different wavelength lasers, and different incident angles.

I hope this helps!
Keith