# Variation of the Double-slit experiment

## Main Question or Discussion Point

Hi Guys,

I am not a physicist, just an average blue collar Joe who finds QM fascinating.

I was wondering why they always use parallel slits. Has anybody ever tried varying the angle between the slits?

And what would the interference pattern look like at different angles, starting at 1 degree, all the way up to 90 degrees.

Just wondering...

## Answers and Replies

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Khashishi
That's a really cool idea. You would essentially be changing the distance between the slits, but viewed across space. So the distance between the bands would vary from top to bottom.

Small supplemental if I may. I understand that the distance between the slits is important, and thus I would suggest 2 possible variations.

1. Imagine a slit which starts as a line, and at 90 degrees forms a L

2. Imagine a slit which starts as a line, and ends up as a +

Now with option 1 you would have to move the bottom part 179 degrees to get the full range of possible refractions, whereas with option 2 you would ony have to move 90 degrees.

Has anybody ever done this? Are there any images of the resulting refraction pattern you would get? I would love to see it.

Hi Just Wondering, and welcome to Physics Forums!

Yes, you can get various "weird" looking interference patterns by changing the slit layout. You could check out this blog post by me - look at picture 4 which shows irregular interference pattern from shining laser light through a thin black cloth (irregular vertical/horizontal pattern since the threads in the cloth are not perfectly parallel).

Note: this was just a quick post and I won't be able to respond until later - I'm going away for a couple of days. But if I remember correctly, there are other pictures of various interference patterns on the internet.

Thank you Dennis. Do you maybe have better pictures of the patterns? I would really appreciate it if you would post them here or send them to me.

There is one more thing I have been wondering about. In some experiments they send one particle at a time, and still get the interference pattern. What I was wondering, is do they keep count of how many particles is sent through? And if so, how many particles miss both slits, and just reflect back from the thing where the slits are in, or do they all pass through either or both slits? And if this is the case, how do the particles know they are supposed to go through the slits?

.Scott
Homework Helper
Hi Guys,

I am not a physicist, just an average blue collar Joe who finds QM fascinating.

I was wondering why they always use parallel slits. Has anybody ever tried varying the angle between the slits?

And what would the interference pattern look like at different angles, starting at 1 degree, all the way up to 90 degrees.

Just wondering...
Of course, you can use any pattern you want. Not just slits. Not just black and whit - shades of gray can be included. Assuming you're using a LASER, if you replace the slits with a piece of a film containing a hologram, you will project the original holographic image as viewed from that spot on the film.

In general, a "Fourier Transform" of the pattern you use will be projected.

Thank you Dennis. Do you maybe have better pictures of the patterns? I would really appreciate it if you would post them here or send them to me.
Yes, I have somewhat better pictures - I had to resize them in my blog post due to forum image size (and blog text size) restrictions. I will upload a set of my original pictures to flickr and then write a new blog post with all the images present - I'll probably do it today (I should have done it before ), so I will post in this thread when I'm done.

There is one more thing I have been wondering about. In some experiments they send one particle at a time, and still get the interference pattern. What I was wondering, is do they keep count of how many particles is sent through? And if so, how many particles miss both slits, and just reflect back from the thing where the slits are in, or do they all pass through either or both slits?
I'm not 100% sure how it's exactly done (or I do not remember ), but if you remove the slits, the particles ought to hit the screen at one particular point (with a very small position distribution which is due to the source, e.g. electron source), so the experimenter ought to be able to calibrate the detection system. In my next post, I will provide some links/papers to double-slit experiments with particles.

And if this is the case, how do the particles know they are supposed to go through the slits?
The basic slit setup (for light/photons) is basically a non-transparent material with a number of thin slits in it - so the photons either pass the slits or not, that is, detected photons after the slits have passed the slits, and any other photons do not matter (but you can get interference with a single thin barrier too). If you remove the slit setup/barrier, the photons will ideally hit the screen at a single point (with a small position distribution due to the laser source).

The Hitachi double-slit experiment with electrons were made with an

Hitachi DSE page said:
[...] "electron biprism", which consists of two parallel plates and a fine filament at the center.
Web page: http://www.hitachi.com/rd/portal/research/em/doubleslit.html
(there is also a video clip on that page)

Paper: Demonstration of single-electron buildup of an interference pattern (Tonomura et al.)
(see fig 4 for the electron position/counting schematics)

Last edited:
Thank you Dennis. Do you maybe have better pictures of the patterns? I would really appreciate it if you would post them here or send them to me.
Hi again, Just Wondering! I have now updated my blog with better pictures (but they are far from good since 1. my objective was to do the experiment with cheap, minimal equipment, 2. my camera is not good and 3. I am a mediocre photographer ). Picture 4 and 5 show the pattern from a thin black cloth and there are also some photos from other things I tried (CD reflection, two consecutive barriers and a slanted copper wire barrier).

Anyway, here they are:

Pictures 1-9 are here in this post.
1. Basic setup with slits in black aluminium foil.
2. Basic setup with slits in black aluminium foil (closeup).
3. Interference pattern from slits in black aluminium foil.
4. Interference pattern from thin black cloth (in light).
5. Interference pattern from thin black cloth (in darkness).
6. Pattern from a CD (in light).
7. Pattern from a CD (in darkness).
8. Pattern from a CD piece (in light).
9. Pattern from a CD piece (in darkness).

Pictures 10-20 are here in this post.
10. Patterns from a CD projected on a screen (in light).
11. Patterns from a CD projected on a screen (in darkness).
12. Laser setup with two consecutive paper barriers.
13. Interference pattern from two consecutive paper barriers (in light).
14. Interference pattern from two consecutive paper barriers (in darkness).
15. Laser setup with a slanted copper wire barrier.
16. Projection on screen from a slanted copper wire barrier (in darkness, enhanced).
17. Projection on screen from a slanted copper wire barrier (in darkness, more enhanced).
18. My adjustable Lego laser device.
19. My cat waiting for laser spots.

I will most likely do more double-slit experiments in the future, with better laser devices and better slits; I'm thinking of buying a better red laser, a green laser and prefabricated slits from some science shop. If so, I will post info about any new experiments in my blog.

The other links/papers I was thinking about before are about interference of massive particles - electrons and even C60 (Buckminsterfullerene), and here they are: