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 Laser Pointer Forums - Discuss Laser Pointers What to do with your diffraction glasses I: wavelength determination

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08-07-2009, 01:55 PM #1
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dr-ebert
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What to do with your diffraction glasses I: wavelength determination

3 weeks ago I bought a couple of these diffraction glasses from Alexzupinhea. After they arrived, I played around with them a bit and came up with two simple-to-do experiments which may have some practical merit for one or the other. The first one is on how to check your lasers wavelength. The second one is a qualitative assessment of fluorescence spectra.

If you shine your laser through these glasses, the single dot gets split into multiple dots, depending on the line pattern of the diffraction glass. With just vertical lines, you'll get a horizontal series of dots. With a criss-cross set of lines, you'll get a 2-dim array of dots, as seen in Alex' picture in the first post. For the reasons why this happens, check Wikipedia or your favorite physics textbook.

Here are some basic diagrams and formulas:

You get an nth-order diffraction spot when the path length difference between two adjacent slits in the grid is n times the lasers wavelength. See the right-hand part of the following picture. On the left-hand part, you see how the diffraction by an angle shows up as a projection against a wall: if the wall is at a distance D, the diffracted spot will be displaced by distance d from the primary beam.

We now have the necessary formulae. Let's transform them a bit.

d and alpha depend on the wavelength lambda, the other values D and g are independent. If we make the measurement for two wavelengths lambda1 and lambda2, we get two measurements for the spot distance, d1 and d2. Dividing, we get:

Next I shone various laser beams through the glasses and measured the distance of the (horizontal) first-order diffraction spot from the primary (undiffracted spot). With D=480cm, I got the following values for d using a green, red and violet laser:

d(green) = 52.7 cm
d(red) = 65.8 cm
d(violet) = 40.3 cm

Assuming that the 532nm wavelength for the DPSS green is the most reliable, and plugging the values into formula (7), we get this result:

wavelength(red) = 662nm
wavelength(BR) = 408nm

None of the measurements were done with any attempt at great precision. Still, the results are obviously within 1%, showing that the method is workable.

Note that the cos terms, and especially their fraction, is very close to unity, so in an approximation, it can be ignored. Also note that the spot deviations, d, in millimeters, are close to the wavelength in nanometers. If the projection distance had been 486cm instead of 480cm, the correspondence would have been nearly perfect, allowing the direct reading-off of an unknown wavelength.

Here's a picture of the three lasers shining through the glasses at the same time, aligned such that the primary spots overlap, thus showing the different amount of diffraction for the various colors. (The picture isn't too good, as my cellphone camera isn't very suitable for dim light conditions).

Attached Thumbnails

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Last edited by dr-ebert; 08-07-2009 at 02:32 PM.

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08-07-2009, 02:05 PM #2
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ElektroFreak
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Re: What to do with your diffraction glasses I: wavelength determination

^Now that's a worthwhile post.. Very nice info, it's been discussed here several times in the past but without the math to go with it (which is kinda important).. The only thing I would change is your reference. HeNe lasers @ 632.8nm are the definitive wavelength reference. They tend to be even more reliable than 532nm green.

The results are heavily dependent on the number of lines per inch on the diffraction grating and the distance from the diffraction grating to the projection surface.. What are the specs on those gratings?
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Last edited by ElektroFreak; 08-07-2009 at 02:43 PM.

08-07-2009, 03:01 PM #3
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dr-ebert
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Re: What to do with your diffraction glasses I: wavelength determination

Addendum: if you play around with the formulas, you can determine the grid constant, given lambda, d and D. For these glasses, it works out to a grid spacing of 4870nm, or 205 lines/mm, or 5200 lines/inch.

I'll leave it as an exercise for the interested student to find the formula (it's trivial).
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08-11-2009, 04:40 PM #4
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Krutz
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Re: What to do with your diffraction glasses I: wavelength determination

hmm.. i tried this once some time ago. even built a curved projectiontable to have a constant distance to the grid. the grid had a known linenumber, no criss-cross. and still, my numbers were way off, the worse the farther i got up in wavelength. even the reliable 532, 808, 1064 and multitudes of these didnt fit..
thanks for reminding me, i will have a look again at this eventually!

manuel

07-11-2013, 12:46 AM #5
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Alex Watkins
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Re: What to do with your diffraction glasses I: wavelength determination

Is anyone still working on this project? I just bought a laser and pair of diffraction glasses from Prism Glasses, Rave Gloves, Quality LED and Glow Products and want to try some of these experiments. Especially with the glasses. Could anyone say if this laser will have the same effects? Triple Action 3-in-1 Laser (Green)

07-11-2013, 01:08 AM #6
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IsaacT
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Re: What to do with your diffraction glasses I: wavelength determination

Subscribing. When my brain hurts just a little less and I grab some food, I will go through and look at all the math again. It's like an algebraic novel up there, and I just can't handle it at the moment. looking forward to it though! I love laser related formulas and plan on making a small notebook filled with any and all related data.

Holy necropost batman! Didn't realize when I read it that it was from 4 years ago! Still plan on reading up though!
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Last edited by IsaacT; 07-11-2013 at 01:09 AM.

07-11-2013, 01:23 AM #7
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Alex Watkins
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Re: What to do with your diffraction glasses I: wavelength determination

woops! I'm a dummy, I just realized that too. We'll I still can't wait for the glasses!! I totally just bought a pair of these too, I cant wait to see what they do to the laser. I think they will give it more dimension regardless.
3Diffraction Glasses

07-11-2013, 03:42 PM #8
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Leodahsan
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Re: What to do with your diffraction glasses I: wavelength determination

I've been searching for this for a while... Subscribing

08-29-2013, 10:44 AM #9
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The Lightning Stalker
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Re: What to do with your diffraction glasses I: wavelength determination

I've been trying to get a spectrum tonight to try and figure out what frequencies the junk e-Bay and o-like "red" safety glasses actually work at. There have been a lot of problems though. I believe the main problem lies with my diffraction grating. It claims to have 13,500 lines/in (1881.4815nm), but the angles I'm getting are saying otherwise. I don't however have a HeNe reference to figure out exactly what it is. I'm thinking of taking some mercury lines as I do have a GE Sterilamp for erasing EPROMs. That thing is dangerous though. It puts out a lot of hard UV. Does anyone have any better ideas?

Here are some pics of my setup:

General Overview

Closeup of the slit. 2mm gap approx.

This guy was hanging around. I think he's interested in science.

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