Science Buddies: "Ask an Expert"

hi, sorry if i posted this in the wrong place, but while dealing with the data in the "Using a Laser Pointer to Measure the Data Track Spacing on CDs and DVDs" on this site, both me and my friend cannot figure out one thing. When m value on the beams of light, it says (he is the link for quick reference https://www.sciencebuddies.org/science- ... ?from=Home)

"# m is the order of the diffracted ray. The reflected ray (when θm = θi) has order 0 (zero). Rays farther from the normal than the reflected beam have order 1, +2, +3, etc. Rays closer to the normal have order −1, −2, −3, etc. In certain cases, for example very small d, some or all of the negative m orders may actually be diffracted through such a large angle that they are on the same side of the normal as the incident light. When the diffracted beam is on the same side of the normal as the incident light, the angle for the diffracted beam is negative. "

but when looking at the picture that has the data example, it seems to be different. in the picture, the reflected beam is at 20* from the normal (m=0) and when m=+1, the beam is at 48* which is farther from the normal line so it fits the rule. On the other side in the pic, when m=-1, it is at -7* which is closer so it fits the rule, but when m=-2, the beam is at -32* which should be farther then the reflected beam so shouldn't it be m=+1 and the current m=+1 should be +2.?

i guess a more simple way to put it would be is those on the side of the incident beam be labeled at a negative value as they get farther away and there is only a negative m value when a beam is coming of between the reflected beam and the normal line (in the example on the site, one at +5* would be m=-1) and the only postive value is when the beam is on the same side as the reflected beam but farther away? i am trying to get how to label the beams down but i can't seem to do it. thanks.

Hi!
Here are are a couple links to similar experiments, hopefully the directions might be clearer-
http://64.233.169.104/search?q=cache:yS ... cd=6&gl=us
http://htestephanie2.blogspot.com/2006/ ... tific.html --> somebody performed the same experiment
Hope this helps, good luck!

Hi,

It sounds like you guys can find the reflected ray, but are having trouble identifying the orders of the refracted rays. Here is a link to another explanation that might (or might not) be clearer than the experimental writeup:

http://books.google.com/books?id=r8I1gy ... CAcliwC0U4

The simplest answer I can give is that in the example you are describing in your question the positive order is numbered sequentially going counterclockwise from the reflected ray, and negative orders are counted clockwise from the reflected ray. As far as I can tell the experiment writeup is correct, though perhaps confusing.

It will be clearer to you if you can understand the theory behind it. In the reflected direction, the ray is exactly the same distance from the scattering points (the CD tracks). You see other refracted rays when the light scattered from adjacent tracks travels an integer number of light wavelengths shorter or farther to reach the observing position. If this is still clear as mud please talk to your science teacher. A quick explanation using diagrams will probably make it very clear.

This is a very neat experiment. I hope you enjoy doing it.

Best regards,

Barrett Tomlinson

barretttomlinson wrote: The simplest answer I can give is that in the example you are describing in your question the positive order is numbered sequentially going counterclockwise from the reflected ray, and negative orders are counted clockwise from the reflected ray. As far as I can tell the experiment writeup is correct, though perhaps confusing.

Those sites were a big help and that anwser was the best way i have seen it put, thanks a lot.