Using a Laser Pointer to Measure the Data Track Spacing on C

[MAULIN]

is it necessary to get 4 diffracted beams (excluding incident beam) as shown in the picture on this site???

[agm]

Hi Maulin,

Welcome to the forum!

Calculating the spacing from multiple diffracted beams is like measuring anything multiple times -- it reduces the error in your final result. This error could be something like plus or minus a degree from reading the protractor, or something more seriously like thinking you were seeing m = -1, 0, and +1 when you were seeing m = 0, +1, and +2. So yes, you can calculate the spacing with fewer diffracted beams, but it might be a somewhat less accurate. One thing you could do to compensate is change theta_i, the angle of the incident beam -- the result for d should be the same with the set of diffracted beams visible at a different angle.

Hope that helps, and let us know if you have more questions,
Amanda

(for reference to others reading the thread, the experiment is here: http://www.sciencebuddies.org/science-f ... p011.shtml)

[MAULIN]

thank u............amanda!!!!

[MAULIN]

i m doing project on LASERS.........
can any one help me in getting a good 'conclusion on lasers' for my prjct????

[agm]

Hi Maulin,

You can find general info about writing a conclusion here: http://www.sciencebuddies.org/science-f ... ions.shtml

and a guide to the whole science project process here: http://www.sciencebuddies.org/science-f ... ndex.shtml.

A conclusion is something that you write after you've finished doing your experiment; if your "just starting" status is not outdated, it's probably a bit soon to worry about this part. If your teacher is asking for a draft of your paper, perhaps you could outline the kind of information you plan to put in the conclusion and give an example like, "If [example result] is observed, then I could conclude that [example conclusion]."

Good luck,
Amanda

[MAULIN]

actually i wntd a readymade 'conclusion on lasers' if possible..........i hv to submit my prj. in few days!!!

[MAULIN]

i m nt able to get clear diffracted rays as shown in the diag.(in this site)......
i m using a red laser pointer with wavelength of 630-680 nm........is this nt enough???
any other factors which i need to keep in mind?????

[ChrisG]

Hi Maulin,
I combined your 3 threads into one. Please keep future comments about this experiment in this same thread so that we can figure out what you are doing and provide the best help possible.

Regarding providing a ready-made conclusion, sorry, but we can not do that. As Amanda mentioned, the conclusions are written based on your hypothesis and the results of your experiment. If/when you have specific questions about how to synthesize your hypothesis and experimental results into a conclusion, then please provide some details about your hypothesis and results, and then we can probably help.

Regarding the problems you are having with the experiment, do you see any diffracted beams at all? I don't see any problem with the laser you are using. Are the batteries low? The photo on the experiment page has been altered to make the diffracted beams more distinct and obvious, so what you see will not be as clear as what is shown in the photo. Does it help if you move to a darker room where you can see the rays more clearly? If it still seems not to work, you can try to make small adjustments to the experimental setup to see if anything helps to produce clear refracted rays. If you're still stuck, let us know!

Good luck,
chris

[MAULIN]

thnx for the help!!

[ChrisG]

You're welcome. Good luck with the rest of the experiment.

[MAULIN]

i conducted the experimnt on a std. CD........
i got the value 1883.515333 nm
is it acceptable??

[agm]

Hi Maulin,

To start, I will tell you that your result is definitely in the ballpark -- well done! Your procedure and calculations are very likely done correctly, although it's possible that you could change some things to make your measurements a bit more accurate. One thing you should be careful of when reporting results is significant figures, a concept that's related to error. For example, if you reported your result as 1883.5 nm (with no +/-), you would be implying that you're pretty sure the value you measured is closer to 1883.5 than 1883.4 or 1883.6 nm. On the other hand, you might decide to report it as 1900 nm, meaning that it's closer to 1900 than 1800 or 2000 nm.

http://en.wikipedia.org/wiki/Significant_figures

It's even better to explicitly state the error, so you might end up with something like 1883.5 nm +/- 250 nm -- in which case you don't lose much by rounding to 1880 nm +/- 250 nm, because the last few sig figs don't mean much with that much uncertainly. 250 nm is just an example -- you need to estimate the sources of error in your experiment, such as from reading the angle on the protractor, and then propagate them to your final result.

http://en.wikipedia.org/wiki/Measurement_Uncertainty
http://en.wikipedia.org/wiki/Error_propagation

It's also worth googling both of those terms and checking the indices of your science books for different explanations. If the error-propagation math is too heavy, you can also do this: Say you measure the angle as 60 degrees +/- 2 degrees. Do the calculations using 58, 60, and 62 degrees. Your result is the one you got using 60 degrees, but you want to choose an error that sets the upper and lower bounds near the results for 58 and 62 degrees, respectively. It's a little more complicated if your result is the average of calculations from several different reflected beams, but you can at least do this for each calculation individually. If the errors are the same for each beam, the general rule of thumb is that the error in the average will be the error in one measurement divided by the square root of the number of measurements that went into the average -- but you should look this up, try to follow the reasoning behind it, and cite a good source (e.g. a statistical handbook) when you turn in your project.

At this point, you should have a decent idea of how much inaccuracy there is in your result due to error. Don't be discouraged by this -- there is error in every single scientific experiment, and it's impressive that you can measure such a tiny length scale with things around the house! Now, you want to look up the actual value. If it falls within your error bounds, then it means that your result is probably as good as it could be within the limitations of your measurement tools. If it's outside of your error bounds but still pretty close -- i.e. the same order of magnitude -- then it's likely that there's a source of error that you've neglected. If it's wildly different, then you should first check your calculations and then carefully repeat the experiment if necessary.

It's a good idea to estimate the error first, because then you won't be tempted to choose larger than reasonable error in your degree measurement, etc, just to make it work out. And if your 'blind' measurement of the track spacing is accurate within your error bounds, you really have something to be proud of. When you're ready, the track spacing can be found in this article:

http://en.wikipedia.org/wiki/Compact_disc

(As a side note, the Greek letter mu, like a u with a funny tail going down in front, is used as a metric prefix meaning micro-, since m and M are taken for milli- and mega-. Micrometers are also called microns, perhaps because there are instruments called micrometers, though the stress is on the second syllable for the instrument when spoken aloud. Just a heads-up since it took me years to get this straight.)

Best wishes, and let us know how it turns out!

Amanda

[hook93]

hello everyone, i am doing this project for my science fair. I finished the research phases, but now i have the experiment to do which is due jan 18. I have some questions about the project that i didnt understand. First thing is that how do i know which defracted angles are m=1, m=-1 etc.....,,, second thing, i dont get how to replace in the formula the stuff that i get, they say its like this
d = m × λ / (sin θm − sin θi ) but what should i put in sin theta-m,, i only put the angle of the defracted beam such as the angle of m=-1?? or should i also multiply the angle by -1 i know at the m part at the beginning of the formula i should put for example m=-1... and last thing is that when i get the measurements for the angles, i have to do the formula for each measurement and then find the average of all of the d's found??

[kgudger]

Hello and welcome to the forum!

If you look at the experiment figure under "making measurements", you will see the various angles labeled. Please note that the m=0 reflection is the one where the angle of reflection = the angle of incidence. Angles on the same side as the reflected beam are positive m, those on the incident side are negative m.

With regards to the formula, the example states:

For example, for m = −1, and a laser pointer with a wavelength of 655 nm, the formula would be: d = (−1) × 655 / (sin θ−1 − sin θi )

. If you set up your calculator to use degrees for trigonometric functions, then θi is the incident angle in degrees, and θ−1 would be the m=-1 angle in degrees.

With regards to your final question, Yes, you should do the formula for each measurement and

Calculate the average value for each d column, and, separately, for all of the values of d.

.

I hope this helps!
Keith

[hook93]

thank you for your help, but i have another question. Concerning the values that i replace to then find the average of the D's found, i shouldnt replace the m=0 value right,?? since the result would be 0 anyways?? or should i?