How many beams from a 1000 line/mm grating? FREE gratings to the first 6 posters*

[Cyparagon]

It has been insisted to me by one person in particular that a 1000 line/mm grating will produce 6 beams. The math says a 1000 line/mm grating will produce 3 beams above 500nm, and 5 beams below 500nm. My testing agrees. I provided photographic evidence of a green laser producing 3 beams, but I was called a liar and told I was doctoring and misrepresenting photographs. I don't like being called a liar. So I'm taking the scientific approach: peer-review.

Note, the second order (fourth and fifth beams) may be very dim and off at a wide angle on the blues/violets. But they should be there according to the math.

I'll give 4 free gratings each to the first 6 people. Here's how this will work:
1) You must have 100 posts or more (for integrity reasons) and be in the US (for shipping reasons).
2) post in this thread to claim a spot.
3) once 6 people have been selected and confirmed, each will purchase https://www.ebay.com/itm/292403982201
4) PM me a screenshot of the receipt (with personal info blacked out as you see fit) and a PP address.
5) I will deposit $6 as reimbursement.
6) when your gratings arrive, perform tests with various colors and post your results and photographs here. If it is indeed 6 beams and not 3 (or sometimes 5), I want to know, because you may be entitled to a Nobel prize in physics for such a discovery.

7*) if you already have a 1000 line/mm grating and would like to participate, please do! Hell, I'll throw in $3 to the PP of your choice for your trouble if you want. (This part of the offer retracted since the matter appears to be settled)


Edit: List of confirmed claimants (1/6):
RA_pierce

Other:
$3 was donated to the forum on jnrpop's behalf on 9/24

 

[Gazen]

That’s generous.

 

[jnrpop]

Only 3 on my tests

J :beer:

 

[RA_pierce]

I'll play.
I don't remember much from college physics but aren't we asking how many orders will be visible given the wavelength of incident light and line spacing?
PM incoming.

Did a little quick research.
At 650 nm, we should expect one order visible (3 beams: one beam straight through and two first order beams on either side.
At 532 nm, we should expect one order visible (3 beams).
At 400 nm, we should expect two orders visible (5 beams).

According to the math I'm doing, at 500 nm is where the second order should become visible.
It should be fun to see how that turns out with the lasers I have in that range.

 

[paul1598419]

I will say that my tests with a 1000 l/mm grating were done with a blue laser and I saw six dots. But, I have come to realize it was because I have a bay window that takes up a full wall in my living room and one of those beams was seen where the next wall made a 45* angle with the wall I was pointing at. So, this is my admission of my mistake. Since the thread that this was about was talking of a 488nm laser, that was the wavelength I used. Enjoy your experiments.

 

[Mattronium]

Sure.
Just tested a few.

Wavelengths that have three dots: 685nm, 660nm, 635nm, 532nm, 515nm, 502nm.
Wavelengths that have five dots: 495nm, 475nm, 450nm, 405nm.
At 515nm you get 4 dots if you hit the grating maybe 20 deg or more off of perpendicular.
At 502nm you get 3 dots if perfectly perpendicular, but if you hit it at anything but perfectly perpendicular you get 4 dots.

Thanks for the excuse to take my lasers out again.

 

[smallfreak]

I may not aply for the deal, but here is my guess:

Theory permits a second order spot (5 beams) on wavelength <500nm, but real worls physics will not allow that due to geometrical constraints on the grating.

I would not expect any interference beam further off than 45° to the grating normal to each side. To fit two orders within such a 90° cone (5 rays) the wavelength has to be as short as 350nm with a (cheap) 1000lines/mm grating.

Anyone with a 405nm violet Laser and a really good grating may eventually see the second order at 54° off the normal.

IF there is really some second order on a blue (no chance for a green one) laser, it might eventually be too faint to be seen directly. Placing a camera at 63° off the normal with a 445nm Blue might catch it.
---
Edit ... and I already have been proven wrong.

Wavelengths that have five dots: 495nm, 475nm, 450nm, 405nm.
At 502nm you get 3 dots if perfectly perpendicular, but if you hit it at anything but perfectly perpendicular you get 4 dots.

What kind of Grating did you use? Certainly not the one from Post #3

Only 3 on my tests

The picture of the blue laser is too narrow to judge whether the second order spot is produced. With 445nm (guess) it must be at least 4 times the distance of the first order spot = tan(63°)/tan(26.5°)

 

[kecked]

I tried this with argon and see lots of dots that get fainter as they go out up and down so I think this is not a pure grating. Many more than 3 or 6. I think I see three sets of color sets Ie orders on both sides up and down too but not sure. It fills the wall. There could be faint ones I don’t see. I don’t know the ruling of this plastic grating and think it’s 300-600 range. It quite what you ask.

 

[RedCowboy]

I tried this with argon and see lots of dots that get fainter as they go out up and down so I think this is not a pure grating. Many more than 3 or 6. I think I see three sets of color sets Ie orders on both sides up and down too but not sure. It fills the wall. There could be faint ones I don’t see. I don’t know the ruling of this plastic grating and think it’s 300-600 range. It quite what you ask.

Argons are multi wavelength of course and it's got to be a 1000 line per mm grating.

 

[Mattronium]

Smallfreak, actual I was using the standard cheap plastic gratings. The second order for the 502nm for example is very faint (visually ~1% of the main laser strength)

 

[smallfreak]

Smallfreak, actual I was using the standard cheap plastic gratings. The second order for the 502nm for example is very faint (visually ~1% of the main laser strength)

Then you are defying physics, the grating is not 1000 lines/mm or the laser is not 502nm (or has a pretty broad peak around that) - or this is some inner reflection.


At 500nm the second order diffracion is a full 90° from the incomming beam, i.e. fully sideways. At >500nm there is no real solution for a second order diffraction at 1000 lines/mm.

That's how the wavelength is to be measured. Physics is quite stubborn regarding the laws of electromagnetic waves.

Could you take a picture of that, preferably shot into a white and roughly spheric kitchen bowl with the grating just inside the rim? This should show the entire 180° of what comes out of the grating.

When the 502nm Laser has a noticable ammount of variation 5nm up and down, you might get a 497nm second order diffraction at 83°

 

[kecked]

Yea I get that. Was just wondering why the one I have makes a grid pattern. Figured to ask related question.

 

[jnrpop]

Other:
jnrpop entitled to $3 if he chooses to PM a PP address.

Very generous, feel free to donate it to this great forum C :beer:

 

[smallfreak]

Could someone who gets only 3 dots with a blue laser try to flip the grating over and shoot it from the other side?

I have not seen any description how these cheap transmission gratings actually are built, but there „should“ be some structure right on top of one of the surfaces, so there should be one correct light path, with the grated (matte) surface towards the screen and the smooth (shiny) surface towards the light source i.e. lit from the back side.

If it is the other way, the deflected rays have to pass through the plastic carrier, facing refraction and even total reflection at shallow angles at the other side. This might swallow the second order spot.

There seems to be no marking on the frame that shows which one is the grated surface, so the grating might well be „protected“ inside with a coating on both sides, like a DVD. This would make handling easier, but reduces the performance.

I assume you already were aware of that, but it should not harm to note it anyway.

Thinking about all that makes me wonder, how light actually interfers when being deflected INTO the medium. Since lightspeed is lower, the frequency is higher inside the medium, resulting in different harmonic results. A 502nm wave then could show a second order harmonic while inside the medium. That one might eventually show up at the other side.

 

[lasersbee]

Using 1000 line grating...
3 dots on 650nm
3 dots on 532nm
5 dots on 405nm

Using 500 line grating...
5 dots on 650nm
7 dots on 532nm
7 dots on 405nm

The extreme outside dots are hard to see
but they are there.

Sorry... I don't have a low powered Blue.
Don't want to burn my plastic gratings.

Jerry

 

[smallfreak]

It has been insisted to me by one person in particular that a 1000 line/mm grating will produce 6 beams.

If you use a really short wavelenth AND you shoot at an angle, there might pop up an additional point.

The first solution with 6 beams is:

wavelength = 399nm
Angle = 11.5°
Spacing = 1µm (1000 lines/mm)

(m-2 = -86°, m-1 = -37°, m0 = -11.5°, m1 = 11.5°, m2 = 37°, m3 = 86°)

No solution at 400nm or longer and even this is a very delicate setup with two beams almost perpendicular to the grid, so the 11.5° are very delicate too.

You may see more patterns, but that would either be reflections or structures within the particular harmonic order. After all, you can project a complicated hologram consisting a zillion different points, but one can not call that "harmonic orders from a regular line grating", as it is no "regular line grating".

Does anyone with a 399nm Laser want to verify that