CD data track

[kabith]

I do not understand how to do it. Should I shine the laser through the protractor? I am not getting a pattern as shown in the figure. I am using a red laser of wavelength 650 nm.

[deleted-71588]

Looking closely at the picture of the setup and paying particular attention to the cardboard spacers, it appears that a piece of wood box is perpendicular to the CD surface. A piece of quadrule index card is taped to the side of the wood box. Three groups of cardboard spacers are holding the protractor away from the index card and the laser pointer is shining between the protractor and the index card so that it hits the CD directly behind the center of the base of the protractor. I suspect that the laser pointer is angled slightly such that it is farther away from the box at the top that it is where it hits the CD surface so that the split reflected beams shine onto the index card.

This would be consistent with the "i" direction:

Direct the laser pointer beam down the face of the index card, and align the beam with the center of the protractor. You may have to fiddle slightly before you see a diffraction pattern like the one in the photo. Make your adjustments carefully, keeping the beam as close to parallel with the card as possible.

[kabith]

I am not getting the five rays

[deleted-71588]

Are you getting any rays?

[kabith]

Okay I got the angkes. How do I get the data track spacing? I calculated the 4 different d's also. Should the average of the d at m= 1, d at m=2 should be added and combined?

[kabith]

m due date is extended. How do I calculate it?

[kgudger]

Hi - I'm going to try and help answer your question. Looking at the experiment, I see that the equation d = m × λ / (sin θm − sin θi ) (Equation 2) gives the answer for "d", which you have calculated. Did you get values for m = -1 and -2 also? Anyway, once you have the values for d, you can do some statistical analysis on them - calculate the average and standard deviation? Try the experiment variation of using a spreadsheet. A spreadsheet can calculate the values for you automatically and also plot any data.

Did you get very different values for d from each measurement? If so, there may experimental errors in your experiment. Can you find the source of the error? This can add interest to your experiment.

Keith

[kabith]

I did get very varied values

[kgudger]

Could you post your data? How about a picture of the setup with the laser operating? Thanks.

Keith

[kabith]

Extremely, extremely sorry. I tottaly forgot about this. Exams came up in between. Here is the image.

I can mark the angles and calculate the values of d, but how do I get the final value?
Thanks and really sorry about the delay (delay is an understatement).

[deleted-71487]

What kinds of values did you get for the various "d" calculations?

The answer is supposed to be an average of the d values, so if they are too wildly different you probably made an error in calculating them. If that's the case, posting your raw numbers and how you arrived at your "d" calculation will let us help you.

[kabith]

Here are the values
θi 48
θ+1 29
θ+2 49
θ-1 78
θ-2 56

d+1 -2516.1
d+2 112410.5
d-1 -2765.9
d-2 -15135.1

For DVD

[kabith]

I am panicking. This project is required for the 7 marks for the ISC boards.

Here is the image


The one with the black extended line is the incident ray. Is is something wrong with the angle that I am taking?

[kabith]

The point is, I am getting d values, but what after that. What am I supposed to do with the d values to get 1600nm for CD's or 750nm for DVD?

[deleted-71588]

Calm down. You probably have a minor misunderstanding. The angles are supposed to be measured with respect to the normal line (a line perpendicular to the plane of the CD). Looking at your photograph and the numbers you listed, they don't seem to correspond to what I would expect. I suspect that you are taking the numbers directly from the protractor scale (measuring inclination from the plane of the CD instead). Go back and carefully look at the project description to see how Theta arc indications show how the angles are supposed to be measured. You can probably correct your angles by subtracting from 90 degrees if I'm correct.

Once you get that right, you can also mis-identify the "m" number. The m=0 number will be the one where it is identical to the incident angle. The negative m numbers will be less than your incident angle. The Sin Theta.M - Sin Theta.I values for the m=2 and m=-2 should be about twice the ones for m=1 and m=-1. If they aren't, look and find the ones that are.

[deleted-71447]

Hi Kabith,
I agree with Craig - don't panic. You'll get it.
Along the lines of what Craig said, the d values are the values of the track spacing, so it's clear that that the d values you are getting are incorrect. I checked that your reported m and angle values do give the same approximate values of d that you reported, so it appears you are doing the calculations correctly. The problem appears to come from the interpretation of your results. Here are my suggestions:
(1) Please reconsider your interpretation of the m values. From the m values you provided, it seems you have chosen the wrong ray as the reflected ray. The reflected ray (m=0) has the same angle from "normal" (the 90 degree mark) as the incident ray, so, in your photo, the reflected ray is the one farthest to the left. All your diffracted rays appear to be to the right of the reflected ray, in which case the m values should all be negative integers.
(2) Please reconsider your measurements of the angles. Diffracted rays to the right of the normal have negative angles. Some of your rays are to the right of normal, but all of your angles are listed as positive. You might want to review the example in the instructions, section 2b.
(3) In the photo you provided, the rays converge at a point to the left of the normal (the vertical line extending down from the 90 degree mark), and appears to introduce an error of +/- 10 degrees in your angles. Ideally, the laser rays should converge where the protractor rays converge, and if that is not possible because of the dimensions of the protractor, then the laser rays should converge directly below the 90 degree mark. Also, it appears you photographed the setup from above and to the right of the protractor. Try to photograph the setup from directly in front of the setup, and at the same level. Please refer to the instructions section, 2a, 2b, and the "note" in section 2b.
https://www.sciencebuddies.org/science- ... p011.shtml
Looking forward to seeing more.
Chris

[kabith]

The angles are measured with respect to the normal, at 90 degrees. Still, the values are weird.

[deleted-71447]

kabith,
To put it in a nutshell, your reported m values are incorrect and some of your reported angles are incorrect. There are some other secondary problems as well. Details are in my post above. I went ahead and made the correct calculations based on your photo, so I know it can be done.
Good luck!
Chris

[kabith]

Phew, at last. Can this be the correct values?



For CD's, I am getting close to 1550 and DVD's, 742.

[deleted-71447]

Kabith, congrats on your progress. It looks like you have gotten some correct values. Some are still incorrect. For example, what do you make of those negative d values and the very large d values? It seems there are some errors in the interpretation of the m values.

Here is how it works, assuming you have the same general setup as the project example and your previous photo with the incident ray coming in from the right.
m=0 has the same angle as the incident ray from the normal.
m=1 is the first ray to the left of m=0, so the angle should be greater than for m=0
m=2 is the second ray to the left of m=0, so the angle should be greater than for m=1
m=-1 is the first ray to the right of m=0, so the angle should be less than for m=0
m=-2 is the second ray to the right of m=0, so the angle should be less than for m=1
etc.

I hope that helps.
Chris

[kabith]

I do not know what to make of those values.

[deleted-71447]

Those strange looking values (large and negative d values) are the results of errors that I described in my previous post.
Good luck!
Chris

[kabith]

YES. I love you people. These are my new values.






Thanks for all the help.

[deleted-71447]

Congrats! I'm glad it worked out, & good luck at the science fair.
Chris

[kabith]

Thank you very much. This project was not for a science fair, but a requirement for my syllabus board.