# Double slit - do it yourself?

1. Jul 26, 2007

### 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?

2. Jul 26, 2007

### jostpuur

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.

Last edited: Jul 26, 2007
3. Jul 26, 2007

### 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.

4. Jul 26, 2007

### 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.

5. Jul 26, 2007

### sas3

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

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

6. Jul 26, 2007

### olgranpappy

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

7. Jul 26, 2007

### cesiumfrog

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.

8. Jul 27, 2007

### sas3

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.

9. Jul 27, 2007

### 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?

Last edited: Jul 27, 2007
10. Jul 27, 2007

### olgranpappy

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.

Last edited: Jul 27, 2007
11. Jul 27, 2007

### 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 quatum 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.

Last edited: Jul 27, 2007
12. Jul 27, 2007

### country boy

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.

13. Jul 27, 2007

### country boy

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.

14. Jul 27, 2007

### 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.

15. Jul 27, 2007

### Jimmy Snyder

Last edited: Jul 27, 2007
16. Jul 27, 2007

### 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"
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, Im shakespeare )
How this can be accomplished ?

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.

17. Jul 27, 2007

### Llewlyn

You dont need the classical model it's enough a bit of qm. Actually u dont know if the photon pass on the right or on the left and that creates interference.

Ll.

18. Jul 27, 2007

### cesiumfrog

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.

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

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

19. Jul 28, 2007

### technobot

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?

Last edited: Jul 28, 2007
20. Jul 28, 2007