Misc. Can the Double Slit Experiment Be Reproduced at Home with Simple Materials?

[technobot]

Hello, is it possible to reproduce the double slit experiment at home with some easily obtainable equipment?

I was thinking of using one of those laser pointers. Though I'm not sure how to make the slits - would cutting a pair of close lines in a piece of plastic with a sharp knife be good enough?

 

[jostpuur]

Hello, is it possible to reproduce the double slit experiment at home with some easily obtainable equipment?

I was thinking of using one of those laser pointers. Though I'm not sure how to make the slits - would cutting a pair of close lines in a piece of plastic with a sharp knife be good enough?

I've succeeded in a doing home made double slit experiment! Unfortunately I didn't have digicamera back then... I have one now, perhaps I should set up the apparatus again and take a picture.

Making the slits is the most difficult part. Once you have then, a laser pointer works well. I think cutting something for slits is difficult, because you would have to be very accurate with your cuts, although it seems possible at least in principle. I'm sure there's lot of ways of making the slits, but I'll tell the way I did them.

I had a thin thread hanging from a pencil, that I had taped in a horizontal position above two empty 0.5l cola bottles. Well the point is, that I had a thin thread hanging vertically. You can achieve that in many ways.

Then I used soldering tin. The reason for this choice is that soldering tin is very soft (and it happened to be available to me), and I could easily make two about 3-4cm long (almost) straight pieces of them. Then I taped these pieces of tin into the thread, also in vertical position. I taped the ends of tin pieces that were up, and the point is that they were hanging on the left and right sides of the thread. Now as you can guess, no matter how well you attempt to tape them so that they would be perfectly just touching the thread on the whole length, they will only touch it at the top, and lower there are a small slits in between the tins and the thread. Since the thread is very thin, these slits are very close to each other.

If you understood the previous explanation, you can skip this paragraph. If you did not because it was too confusing, I'll "draw a picture" here with coordinates. y-coordinate is vertical (in reality), and x- is horizontal. The thread is a line between points (0,L) and (0,0), the tins were lines between points (-eps,h),(-eps,h-l) and (eps,h),(eps,h-l), where L is the length of the thread, l is length of the tins, h<L, and eps>0 is something small.

If I remember correctly, the distance from these slits to the screen was about 3 meters. A red laser produced an interference pattern, where the peeks were separated with a centimeter or something like that. Anyway, very visible with a naked eye.

 

[olgranpappy]

Shine your laser pointer on the readable side of a CD such that the light reflects onto a white wall or something. you will see more than one spot via diffraction.

 

[country boy]

I've held a piece of hair through the middle of a helium neon laser beam and had great results. I'm not sure if it will work with a laser pointer. If anyone tries it before I do, let me know.

 

[sas3]

Here is a good site for you.
They use a Microscope slide and candle soot.

http://www.altair.org/TwoSlit.html

 

[olgranpappy]

Here is a good site for you.
They use a Microscope slide and candle soot.

http://www.altair.org/TwoSlit.html

um... yeah, great... except for the headline that says, "physics cannot explain how the 2 slit experiment works." WTF?

 

[cesiumfrog]

I've held a piece of hair through the middle of a helium neon laser beam and had great results.

Yep, using that with a cheap laser pointer is the simplest way, though some may want some "Fourier grammar" to fully understand why the obstruction is equivalent to a slit aperture.

 

[sas3]

um... yeah, great... except for the headline that says, "physics cannot explain how the 2 slit experiment works." WTF?

I didn't read the whole page. I just thought the idea for the slits was good, but now I wonder what effect the glass would have on the pattern.

 

[technobot]

Ok, thanks for the suggestions - keep 'em coming. Btw, anyone tried to fill the area between the slits and wall with steam to see the diffraction pattern form along the way? And just a safety note for that - the scattered light would be weak enough to be harmless, right? Or would I need something like sunglasses?

One more thing - is there an optimum size for those slits, or would anything under, say, a millimeter would do?

 

[olgranpappy]

...And just a safety note for that - the scattered light would be weak enough to be harmless, right?

Just don't direct the incident laser pointer beam (or the diffracted laser pointer beams) at anyone's eye for a prolonged period of time. [or any period of time, really].

Other than that, I'm sure you'll be fine.

 

[DaveC426913]

Last month's SciAm has an article showing exactly how to make the quantum eraser experiment for just a few bucks. You need a laser pointer and a small sheet of polarizing film and that's it. It uses a single thin wire for the "slits".

I went out and got the parts but haven't assembled them yet.

They supply a caveat: they are careful to point out that diffraction can be explained by wavelengths of light from the laser alone - it is the fact that these diffraction patterns will appear even for a single photon at a time that is the weird part about quantum physics. But the experiment (more specifically, the laser) cannot emit photons one at a time. So it is not a true demonstration of the effect.

 

[country boy]

Yep, using that with a cheap laser pointer is the simplest way, though some may want some "Fourier grammar" to fully understand why the obstruction is equivalent to a slit aperture.

I assume that the hair divides the laser beam into two "slits." Not sure what happens as the beam diameter increases. That's why I wasn't sure about using a laser pointer, which can have a larger beam than a HeNe laser. If it does indeed work, that makes the demonstration a lot easier.

 

[country boy]

Shine your laser pointer on the readable side of a CD such that the light reflects onto a white wall or something. you will see more than one spot via diffraction.

This is a cool experiment that I've used with students. You can even derive the CD "groove" spacing if you know the laser wavelength. Once you know the CD spacing, you can find the wavelength of a different color laser.

 

[ArielGenesis]

I did that one with microscope slide and candle soot, also with CD. And both succeded. I did it in school as one of my lab. with CD it was very easy, with candle soot, it was a bit difficult. I also did it with sound by using two big speaker distanced few meters apart in open areas. And the most interesting one was with water. In a tank of water, a vibrator that vibrate in phase was put inside the water. the experiment was done in a dark room, using stroboscope lamp (the one that blink very quickly). when the blink was in phase with the vibration, we felt like we just stop the time.

 

[Jimmy Snyder]

If you hold your hand about 6 inches from your face and look at a light through the spaces between your fingers, you can see a diffraction pattern.

http://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.html

 

[alvaros]

I think that the main idea behind the double slit experiment is to prove the interference of a single photon. ( this experiment can be made with electrons but you are not talking about it )

From "http://physicsweb.org/articles/world/15/9/1"

In 1909 Geoffrey Ingram (G I) Taylor conducted an experiment in which he showed that even the feeblest light source - equivalent to "a candle burning at a distance slightly exceeding a mile" - could lead to interference fringes. This led to Dirac's famous statement that "each photon then interferes only with itself".

So you need to reduce the power of ligth reaching the double slit to a level that just a single photon reaches the double slit. ( yes, I am shakespeare )
How this can be accomplished ?

Not directly related to your thread but very important to me:
And a question that now arises is how you calculate the time a photon last ( or how long is it ) to be sure that there is just a photon at a time.

 

[Llewlyn]

Yep, using that with a cheap laser pointer is the simplest way, though some may want some "Fourier grammar" to fully understand why the obstruction is equivalent to a slit aperture.

You don't need the classical model it's enough a bit of qm. Actually u don't know if the photon pass on the right or on the left and that creates interference.

Ll.

 

[cesiumfrog]

I assume that the hair divides the laser beam into two "slits." Not sure what happens as the beam diameter increases. That's why I wasn't sure about using a laser pointer, which can have a larger beam than a HeNe laser. If it does indeed work, that makes the demonstration a lot easier.

It'll only take a moment, so verify it yourself rather than taking my word. That interpretation is a bit too simple, because it would lead you to expect the pattern to depend on the width of the laser beam, and wouldn't really explain why using *two hairs* gives the characteristic *double slit* interference pattern.

Last month's SciAm has an article showing exactly how to make the quantum eraser experiment for just a few bucks.

Very neat. I'm forwarding it to the first year coordinator here.

If you hold your hand about 6 inches from your face and look at a light through the spaces between your fingers, you can see a diffraction pattern.

That one's new to me as well. Hmm..

 

[technobot]

If you hold your hand about 6 inches from your face and look at a light through the spaces between your fingers, you can see a diffraction pattern.

Yea, you may be able to see some diffraction (as in light going around your fingers), but if I'm not mistaken you won't see any interference (as in light adding up constructively and destructively to form those familiar patterns). AFAIK you'll need a coherent beam such as a laser to see the latter, and I wouldn't advise you too look at a laser beam through the spaces between your fingers.


Btw, any way one could produce a single photon (or electron, or whatever) at a time with (relatively) easily obtainable equipment or components? And I guess you'd also need some special detection equipment for that too, like maybe a high sensitivity photographic film and a darkroom (or at least a dark-box?

 

[cesiumfrog]

AFAIK you'll need a coherent beam such as a laser to see [interference pattern effects]

Google Newton's rings.

 

[jostpuur]

If you hold your hand about 6 inches from your face and look at a light through the spaces between your fingers, you can see a diffraction pattern.

http://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.html

I know this phenomena, but I haven't been sure what it really is. Is it really interference of light, or just something else?

techonobot, have you seen the lines between your fingers yourself? Do you know what they are?

The message of figure 1 of that web page contradicted my understanding of sun light and clouds. I thought those light beams from sun are all parallel, but only appear to spread because of the perspective, and because of the disability of human eye to recognize it due to the large distances. And indeed, if you look carefully, you will see that all the lines are coming directly from the sun.

 

[jostpuur]

Google Newton's rings.

Somebody once said me that sure interference can be achieved with white light, and showed me a black and white picture of Newton's rings. But now Wikipedia explains this:

When viewed with a monochromatic light it appears as a series of concentric, alternating light and dark rings centered at the point of contact between the two surfaces. When viewed with white light, it forms a concentric ring pattern of rainbow colors because the different wavelengths of light interfere at different thicknesses of the air layer between the surfaces.

So does this mean, that if you are not seeing rainbow colors, you are not seeing interference with white light? Sounds quite correct to me, and I wasn't seeing rainbow colors in between my fingers.

Or perhaps there was rainbow colors coming from between my fingers, but my eye wasn't accurate enough to see the colors?

http://en.wikipedia.org/wiki/Newton's_rings

 

[technobot]

Google Newton's rings.

Somebody once said me that sure interference can be achieved with white light, and showed me a black and white picture of Newton's rings. But now Wikipedia explains this:

When viewed with a monochromatic light it appears as a series of concentric, alternating light and dark rings centered at the point of contact between the two surfaces. When viewed with white light, it forms a concentric ring pattern of rainbow colors because the different wavelengths of light interfere at different thicknesses of the air layer between the surfaces.

There's also a difference between monochromacity and coherence. Monochromacity is when the light is all the same wavelength. Coherence is when it is all in the same phase. A laser has both of these qualites, whereas typical white light (e.g. from a lightbulb) has neither. And when I said one needs coherent light to see interference, I was referring mostly to an experiment similar to double-slit. Because there, if your beam is incoherent (even if it is monochromatic), you'd get contributions from all the phases, so you won't get the distinctive constructive/destructive patterns. Whereas with a coherent beam, the path length difference becomes an issue, so you get those patterns.

I'm familiar with Newton's rings. Though I admit I rather forgot about them. But if I'm not mistaken, in the case of Newton's rings, you actually get standing waves because of the reflection part, so coherence is no longer an issue. And then (I think), the rings form because of ability or lack thereof of making a standing wave of some given wavelength in a given interval - the interval length needs to be a multiple of half the wavelength. (and PLEASE somebody correct me if I'm talking nonsense here, because it has been quite a while since we studied this...).

Btw, a standing wave is also due to interference - of two waves of the same wavelength moving in opposite directions (such as the incident and reflected rays).

techonobot, have you seen the lines between your fingers yourself? Do you know what they are? [...] So does this mean, that if you are not seeing rainbow colors, you are not seeing interference with white light? Sounds quite correct to me. And I wasn't seeing rainbow colors in between my fingers.

Just tried it myself. Actually could see a bit of a rainbow halo - but just barely. So I guess there is a bit of interference there after all, but mostly it's just diffraction without interference.

 

[country boy]

...and wouldn't really explain why using *two hairs* gives the characteristic *double slit* interference pattern.

I believe that's the inverse diffraction pattern. The light and dark stripes trade places.

 

[country boy]

So I guess there is a bit of interference there after all, but mostly it's just diffraction without interference.

How do you get diffraction without interference? Even a single slit diffraction pattern is the result of constructive and distructive interference.

The interesting thing (as you pointed our earlier) about the two-finger experiment is that it can be seen in white light.

Note: it works with edges of paper, too, and is easier to control. Plus, a single edge shows the effect.

 

[country boy]

If you hold your hand about 6 inches from your face and look at a light through the spaces between your fingers, you can see a diffraction pattern.

http://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.html

Okay, I've pretty much decided that this two-finger effect is not a diffraction or interference effect. Here's why:

1) Forming a tapered slit from the edges of two pieces of paper, I can angle them so that the "fringes" between the edges converge. Some pairs of dark stripes merge into a single stripe. That can't happen with real interference fringes.

2) With a single edge, I see double or multiple edges in the out-of-focus image. The edges are less distinct when viewed with one eye than the other. That seems to mean that the effect is related to the eye, not to the paper edge.

3) I made a small round hole and tried the experiment. When viewed out of focus there is a pattern of dark lines and spots that I assume is the same effect as the dark stripes in the edge case. Here's the clincher: when I rotate the hole, parts of the pattern stay fixed with respect to my eye. Rotating my head instead of the hole gives the same result. The pattern is related to my eye, not to the hole or the background lamp.

My conclusion is this. The eye sees an out-of focus image of the edge as not smooth, but as having distinct sub-images that appear as multiple edges. This might be caused by "pinhole camera" sub-aperturing in the eye lens, but I'm not sure if that's what's happening here. When the images of two out-of-focus edges overlap, a combined pattern of the multiple edge images is formed. It might be fair to call this a Moire pattern. When a pinhole is used, multiple circular edges are formed in the out-of-focus image. These overlap to form spots and lines in a variety of positions and directions that are fixed with respect to the eye.

So the effect seems to be due to the eye and not to diffraction at the edges of the paper. I'd appreciate hearing from anyone who has tried similar experiments.

 

[technobot]

How do you get diffraction without interference? Even a single slit diffraction pattern is the result of constructive and distructive interference.

In hebrew there is a separate word for the part where the wave goes around an obstacle, and another word for the part where two waves combine and either add up or cancel each other. When I say "diffraction", I mean the former (going around obstacles), and when I say "interference" I mean the latter (combining). Perhaps I'm misusing the terms?

Also, I'm not entirely sure that what I did is what jimmysnyder meant. What I did was this: I held my hand at almost arm's length against a bright source of light (in my case, the reflection of the sun from some very specular object some 20-30 meters away). I held my fingers almost completely next to each other, and aimed my hand such that the light would be partly blocked by my fingers and palm. Then I closed my right eye and looked at the light with my left eye, via the space between my fingers. What I saw was lines of light over my palm and fingers, where supposedly the light was blocked, plus a very weak circular rainbow-colored halo around the light, with the color lines forming incomplete concentric circles around the light. They were incomplete because I could only see them on the background of my hand's silhouette. If it makes any difference, I was indoors.

This seems like diffraction to me, but after reading your posts I suppose it might be just a lens flare in my eye, or something of that sort...

 

[country boy]

In hebrew there is a separate word for the part where the wave goes around an obstacle, and another word for the part where two waves combine and either add up or cancel each other. When I say "diffraction", I mean the former (going around obstacles), and when I say "interference" I mean the latter (combining). Perhaps I'm misusing the terms?

Fair enough. I was just using the generalization that where light goes is always determened by wave interference.

As an example of my usage, a "diffraction grating" uses the interference from scattering at grating rulings to send light in various directions. I would say this is what the CD is doing in a previous post.

 

[vijayan_t]

I have done successfully
see here

 

[Pitullie Paul]

Please help, how wide should the slit be and how far apart.

 

[YuriKolovsky]

@country boy
I know this reply is 6 years late, but I just tried it just now with a cheap laster pointer that is built into a pen and a thick piece of hair and after a while of positioning was successful. just to let you and others know.

 

[Integral]

Please help, how wide should the slit be and how far apart.

As close together and as narrow as you can make them. It is easy to make them too wide and too far apart, very difficult to get them close enough and narrow enough.

 

[Pitullie Paul]

I was successful in the end with a cheap pen laser and slots cut in tin foil (aluminium foil). I cut the slots with a scalpel then closed then again to make them small. Lots of fun.