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Experiment : Double Slit Answered

Hi !

I tried to reproduce the Double-Slit experiment with a cheap red laser-diode (<1mW).

Below are various results obtained from some variations of the experiment ... (with a double slit, with a pinhole, with a thin wire, and with a hair (and also with a single slit who gave almost the same result than with a hair))

I also tried to reproduce it with an other source of light (and got electrocuted). But so far, I'm not satisfied by the results I obtained with the light bulb, and I failed with the sun.

Do you have some experience to share about this experiment ?
Do you have successfully reproduced it with an other source of light than a cheap laser diode ?

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kevinstuartfr0st (author)2010-02-22

http://www.youtube.com/watch?v=2KD9ICmaiGs#
this is a video of what i think is going on with double slit,first you have to relise that we look at a small cross section of what i think is there,in my video i try to show the bigger picture easer to look at video than explain but in a few words,2 newton rings with the inner edge pinched together, and take a small cross section,lol said isnt easy to explain my video should help
comments welcome
kevinstuartfr0st

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chooseausername (author)2007-10-12

They are very boring ........ :-/ I've spent the two last hours to quiz Google ....... I'm unable to find any picture showing the actual results of the experiment with a NON coherent source of light (sun or light-bulb). They will show you results obtained with lasers, with electrons, with "water", they will give you simulations, theories, tons of blabla, equations, drawings and even fake 3D illustrations, but NONE OF THEM seems to actually have reproduced the original Young's experimentation ... So, how could I compare my potentially wrong results with scientifically validated results if no one provided any ??? Well ......... that's not serious ...

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Kiteman (author)chooseausername2007-10-12

...the original Young's experimentation...

Unfortunately, according to Wiki:

Tony Rothman in Everything's Relative and Other Fables from Science and Technology argues that there is no clear evidence that Young actually did the experiment.

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chooseausername (author)Kiteman2007-10-13

... I picked-up an experiment in the middle of thousands, and this experiment is probably just a legend. How lucky am I ? ... It's a great disappointment. Thanks Kiteman. I did not read the English version of this article ... In the French one, Tony Rothman is not mentioned. This morning, I've made a hole into the shutter of the window, and tried with a ray of sun : it did not work. I did not even got something close to what I got with the light-bulb ... (sun rays are highly paralyzed compared to light-bulbs rays. It should have worked like with a laser ?) I will study a little bit more the theories behind all of that ...

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Kiteman (author)chooseausername2007-10-13

The english word is polarised (all waves vibrating in the same plane). Paralysed = unable to move (for example, because of a broken back).

Sunlight is not polarised in one plane - it has light vibrating in very plane ("circular polarisation").

Polarising the light might help to get the desired results - either pass the sunlight through a polarising ("Polaroid") lens, or reflect it off a mirror at a low angle. Once you've polarised the light, pass it through a slight that is perpendicular to the plane of polarisation.

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Kiteman (author)Kiteman2007-10-13

Oops - messed up my formatting:

The english word is polarised (all waves vibrating in the same plane). Paralysed = unable to move (for example, because of a broken back).

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chooseausername (author)Kiteman2007-10-13

Oh ! I did not wanted to write "paralyzed", but "parallelized". I did not notice it ...

Yes, I wanted to say "parallelized", like the light of a laser beam =o)

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Kiteman (author)chooseausername2007-10-13

Ah, I see. In that case, the phrase is "much more parallel". Parallelised is not (currently) a real word.

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chooseausername (author)Kiteman2007-10-13

Thanks =o)

I will try again tomorrow (if the sun comes to the rendezvous) with a polarized filter (they are old, damaged and low quality, but I don't have better) ...

Also, I was thinking about adding a color filter to reduce the spectrum, but I don't have any of it ... (I will try with a flattened drop of ink between two microscope slide, but I doubt that will do the work ...)

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Patrik (author)chooseausername2008-05-19

I don't think you need polarized light, but it should definitely be monochromatic to get a clear interference pattern. Try splitting a sunbeam with a prism.

By the way, here is a copy of Young's original paper:

Experiments and Calculations Relative to Physical Optics (page 68-76).

Note that he mentions seeing "fringes of color" - a clear indication he wasn't using monochromatic light. The interference pattern for each frequency is proportional to the wavelength of the light, so if you use light that is not monochromatic, the different interference patterns will overlap each other. So instead of a clear dark-light pattern, you get bands of different colors.

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chooseausername (author)Patrik2008-05-20

Thanks for the link to the Young's original paper !

As my goal was to actually try to see what Young saw, I don't mind to use polychromatic light =o)

I've just reproduced the experiment with a different setup than the one I used last year. The position of the sun allowed me to simplify the installation (no need to struggle with a mirror).
I've successfully observed and photographed the fringes on a shadowed edge and a shadowed corner.
I also observed the diffraction through a single slit.

Unfortunately, I did not go farther as the sun moved quickly and some clouds went to sabotage my efforts ... (not to mention how unstable was my setup).

=o)

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Because of my roof-window, I'm afraid experiments involving sun have to be postponed to next summer ...

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Goodhart (author)Kiteman2008-05-19

It may not be a real word now, but all he has to do is go to Los Angeles and speak it out loud for a few days, and it will appear in one of the dictionaries here LOL

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lenoxlaser (author)2008-05-19

We actually produce young's double slit and the precision slits are cut with a laser. Our engineering department has some great photos of the slits and optical signatures. Contact me with any questions you have.

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Thanks for your proposal and for bringing this topic to the surface. That was a last year experimental hobby. My goal was to try to see what Young saw himself. This afternoon I've reproduced the experiment again, using a different setup (the position of the sun allowed a simpler installation this time). And I successfully observed the fringes on a shadow edge and corner, and also what seems to be a diffraction through a single slit ! (unfortunately, impossible to take a picture of it ...) Has your engineering department published some pictures on the internet ?

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zachninme (author)2007-10-09

I'm very surprised it worked, I assumed you needed a super-ultra-focused-expensive laser to do it... good job! Now, where's the instructable? :P

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When made with a laser, it's so basic that I don't think it deserve an instructable ;-) Maybe I'll do one when I'll successfully have made one with an other source of light ...

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. Well, I'm impressed as hell and think it does deserve an iBle.
. I've never actually done the experiments, but from what I've read, if you want to use non-coherent light, then the slits need to be as narrow as possible (thus, longer exposure times) and as precise as possible.
. When using a coherent light source, the optical bench needs to be as vibration-free as possible. I've seen heavy stone blocks, resting on springs, used for making holograms. It might be worthwhile to check some of the hologram sites or even audiophile sites (especially those dealing with turntables) for ideas on how to reduce vibration.
. Using mirrors will increase the path traveled and enhance the effect.
. If you can figure out how to do the one-photon-at-a-time experiments, that will be uber-cool.

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. Well, I'm impressed as hell and think it does deserve an iBle.
It looks probably more impressing than it is actually ... It's very basic ...
You just need a laser, a hair from your scalp, and a wall.
You switch on the laser and "split" the ray with your hair and voila ... You get a divergence (that's how it worked with me though)

If you cast the beam near the hair, you'll see the shadow of the hair.
If you cast the beam farther, you'll see something similar to my pictures ...

The same experiment with a non-coherent source would be more impressing (as it requires some more work).

Using mirrors will increase the path traveled and enhance the effect.
Actually, mirrors (at least the mirrors I use) give a side effect.
The laser beam is reflected twice : once by the tin, and once by the surface of the glass ... you get some "ghost".

If you can figure out how to do the one-photon-at-a-time experiments, that will be uber-cool.
lol, the distance between this basic experiment and one involving photon control is a way too long for me ...
=o)

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Kiteman (author)chooseausername2007-10-12

Even though you think it's basic, I still think it's worth an 'ible, especially if you discuss some of the science and show other versions (like the ripple-tank).

If you want to avoid the double-reflection of the mirrors, get hold of surface silvered or surface coated mirrors - they reflect from a silvered layer on the front of the glass, and no light passes through the glass.

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chooseausername (author)Kiteman2007-10-12

Even though you think it's basic, I still think it's worth an 'ible, especially if you discuss some of the science and show other versions (like the ripple-tank).

Ok. Maybe I'll do later. But I absolutely want to reproduce the original Young's version of the experiment, before.

I tried to reproduce it this afternoon with a 2600 lumen halogen light bulb.
But the results I got are too far from what "everybody" on the internet describe with theories and drawings .... (makes me thinking about the abominable snowman : few claims they saw it, everybody talks about it and even draw it, but there is no actual pictures of it).
So I have absolutely no reference to compare my results with ... and that makes me very frustrated ... :-/

Here is the best result I got so far. But it's nothing obvious like Young's is supposed to have seen ........

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Goodhart (author)chooseausername2007-10-10

You just need a laser, a hair from your scalp, and a wall.

Improvement then, could come simply in the form of using a "splitter" that was less translucent (hair). Maybe a piece of thin coil wire painted black?

Another thing, although this drives expense up a bit, is to get front surface reflection mirrors. This way, the light is not refracted while going through glass twice (on the way in, and on the way out of each mirror).

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Goodhart (author)NachoMahma2007-10-09

to get the single photon effect, don't you have to induce "tunneling"? That would get difficult at room temperature I would think.

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NachoMahma (author)Goodhart2007-10-09

. Not sure how it's done, but assume it would be way out of reach for a DIYer.

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Goodhart (author)NachoMahma2007-10-09

Yeah, a quantum "gate" is set up allowing only one photon or electron through at a time. Without the proper equipment, it is only something "neat" to think about :-)

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Actually, it was so easy, that I wonder if what I saw is what I think .... If I could put my hands on a more serious laser, that would help a lot to figure out ...

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Kiteman (author)2007-10-09

The double-slit experiment always gets best results with the purest monochrome light - if you use a range of frequencies (such as sunlight or the light from a tungsten-filament bulb), then you are using a range of colours. Each colour will get diffracted by a different amount, so the dark bands of one colour will be filled by the bright bands of another colour.

Obviously, solid-state light sources are the easiest way to get a pure monochrome, and lasers give the brightest beam, so a laser LED is your best bet.

You will also get more dramatic diffraction if the gap-size is closer to the wavelength of the light. Razor-blades are good for this, but be careful (duh!), or use a microscope slide covered in opaque lacquer (or maybe black marker will do?), and incise gaps in it with a new craft-knife blade - you may, with extreme care, be able to produce a diffraction grating this way.

Or try try reflecting the beam off a CD at a shallow angle, like this?

If you're working on this as a school project, try comparing laser diodes - do all "red lasers" produce the same dispersal pattern from an otherwise-identical set-up? If not, why not? How do they compare to lasers ripped out of old CD/DVD players and ROM-drives? Could you get hold of a blue-ray laser and an old-format red laser and find out why the blue laser is "better"?

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chooseausername (author)Kiteman2007-10-09

Thanks Kiteman =o)

Actually, it's not a school project. I just try to reproduce some "basic" and "fundamental" experiments to see by myself what scientists from the past saw ...
(That's why I would like to make it with the sun or a light-bulb too, as T.Young did not have access to lasers). =o)

I tried with two different red laser diodes and got very similar results.
Though, I did not try with "identical" setup as my bench and my equipment is "improvised" and not very stable, and my budget very limited ...
I guess I will not have access to a blue or a green laser ...

I will try to improve the slits and my equipment though ...

Also, I found that it is possible to isolate one of the "rays" and to split it again.
This works horizontally, vertically and in diagonal ...

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Kiteman (author)chooseausername2007-10-09

Maybe borrow some welding kit and strike an arc as your light source (don't look directly at it, though!)? I'm impressed that you're doing this as a hobby - you ought to write up your findings, include photos of your set-up, and hand it all to a teacher, try for extra credit on your scores (if you're year 10+, you'll be expected to write up an investigation at some point, why not one of your own choosing?).

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chooseausername (author)Kiteman2007-10-09

I know that my mastery of your language (and my schoolkid humour) makes me seems younger than my actual age, but yes, I'm largely 10+ =o)
And I've been forced to cancel study some years ago ... sadly :-/

lol
Just by curiosity : how old do you think I am ???

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Kiteman (author)chooseausername2007-10-09

Sorry - I tend to assume that most people here are about 18 going on 12. I also forgot you weren't British - Year 10 in the UK are aged 15. Any guess would be a complete stab in the dark...

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chooseausername (author)Kiteman2007-10-09

Ah ok ! "year 10+" is a grade !
I thought it means "slightly more than 10 years old" lol

Also, I got some difficulties to understand the meaning of "try for extra credit on your scores", but I think I got it right.

Any guess would be a complete stab in the dark...
Just a quick dichotomy to add some penumbra into the darkness (but don't stab me please if you see my shape !) : I'm younger than you and older than 18 ... ;-)

=o)

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chooseausername (author)Kiteman2007-10-09

You took the middle point, didn't you ? =o)

If so, you must be 40 ... Doh ! Stupid me, I've just remembered you told it in a previous "birthday thread", lol

29?
less =o)

Wow ... that flu exhausts me ... time to sleep now ...
Good night !

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chooseausername (author)Kiteman2007-10-10

Hey ! I said "penumbra" ! Not "spotlights" !

But if you're afraid of penumbra, here is something for you. =o)

(more ...)

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. Do you have any pics of the splitting? Would love to see them. . What are the blue squares?

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Do you have any pics of the splitting? Would love to see them.
No I don't. I will try to reproduce it again with a better improvised equipment ...

. What are the blue squares?
They are simple 'occluder' ( mainly pieces of paper or plastic ) to block rays you you don't want.

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