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View Full Version : How to set up the two slit experiment



coliver
2009-Dec-29, 12:42 PM
If one wanted to set up the two slit experiment, what would be needed to duplicate the original? What type of light source, backing etc?

WaxRubiks
2009-Dec-29, 01:29 PM
I made one in my living room once, from a a laser pointer and a bit of card with slits cut into it.

ShinAce
2009-Dec-29, 04:17 PM
I made one in my living room once, from a a laser pointer and a bit of card with slits cut into it.

Ditto! Laser pointer held down in a shoebox with tape. Laser pointer button held 'on' with tape as well.

Cut a quarter sized hole in the box and then taped foil in its place. I used a thin sewing needle to punhc two tiny holes no more than 1 mm apart and set the box about 10 feet back from the wall. Turn off the lights and enjoy.

If the holes are too big or too irregular, you can get some funky diffraction patterns from edge diffraction.

edit: check out http://www.bautforum.com/space-astronomy-questions-answers/81076-double-slit-experiment.html

ShinAce
2009-Dec-29, 04:34 PM
p.s. I did not use slits as I wasn't interested in clamping razor blades together in a fancy homemade jig.

The laser is plenty strong enough to use point sources instead of line sources.

OldAstrofan
2009-Dec-29, 10:30 PM
This might be interesting for you
http://corticalcafe.com/prog_CGHmaker.htm
you could create all kinds of patterents including double slits with your printer

as for the light source, i suggest you could try out laser diodes that already have a foucusing lens attatched but without any circuitry, that way you could have a adjustable light source (i'd say try out 5mW laser diodes, they could actually go up to more than 8mW (the working current of 5mW is only 20mA at ~2V))

ShinAce
2009-Dec-30, 01:06 AM
If one wanted to set up the two slit experiment, what would be needed to duplicate the original? What type of light source, backing etc?

I'm sorry, just noticed we haven't answered your question.

If you want to duplicate the original, you'll need the sun beaming in your window, heavy curtains, some paper, some way of making slits, and a nice long path. Close off any light from the room, now open a small gap in the curtain, and tie/tape a paper towel tube. Do not ever look through the tube. Line it up so that it makes the strongest spot of light on your hand/screen and secure it in that position. Now be ready cause the sun does move. Seal off the end you still have access to with paper and poke a small hole, maybe 1/4"(5mm). Place a piece of paper, roughly a foot away, perpendicular(you could measure it using the 4 strings of the same length from the tube to the paper corners and pulling both the strings and paper taught) to the tube and make your slits less than 1mm apart and very fine. I prefer making holes in foil, so use that if you want. Now place your screen as far away as possible and cover up any leaking light. If it's done right, you'll get your pattern

coliver
2009-Dec-30, 01:42 AM
Thanks for the great ways to set it up, this should be most helpful. Another question I had was how could you measure the results or actually count the photons? Or would you have to have an electron gun or other specialized equipment to do that type of experiment? I'm thinking more along the lines of the one that allows variable amounts of electrons or photons to be directed at the backing plate? I guess its a variation of the two slit experiment? Anyone know where to buy equipment like detectors etc?

StupendousMan
2009-Dec-30, 06:53 PM
Another question I had was how could you measure the results or actually count the photons? Or would you have to have an electron gun or other specialized equipment to do that type of experiment? I'm thinking more along the lines of the one that allows variable amounts of electrons or photons to be directed at the backing plate? I guess its a variation of the two slit experiment? Anyone know where to buy equipment like detectors etc?

TeachSpin (http://www.teachspin.com/) sells equipment which will permit you to count the individual photons after they have passed through two slits. In fact, you can set up their equipment so that only one photon at a time is passing through the slit(s), and STILL measure the interference pattern.

We use their equipment in our Modern Physics Lab course here at RIT ...

Beware -- the PMT makes it pretty expensive.

coliver
2009-Dec-31, 12:51 AM
TeachSpin (http://www.teachspin.com/) sells equipment which will permit you to count the individual photons after they have passed through two slits. In fact, you can set up their equipment so that only one photon at a time is passing through the slit(s), and STILL measure the interference pattern.

We use their equipment in our Modern Physics Lab course here at RIT ...

Beware -- the PMT makes it pretty expensive.

Great stuff, but not for the beer budget..:) I see what you mean. I would like to get the separate components rather than a complete unit, they have a nice one though.

ShinAce
2009-Dec-31, 05:26 PM
TeachSpin (http://www.teachspin.com/) sells equipment which will permit you to count the individual photons after they have passed through two slits. In fact, you can set up their equipment so that only one photon at a time is passing through the slit(s), and STILL measure the interference pattern.

We use their equipment in our Modern Physics Lab course here at RIT ...

Beware -- the PMT makes it pretty expensive.

PMT: photo multiplier tube???

I thought that's what my retina was for! Or better yet, rhodopsin...

StupendousMan
2009-Dec-31, 08:13 PM
PMT: photo multiplier tube???

I thought that's what my retina was for! Or better yet, rhodopsin...

PMTs are better than retinas for making quantitative measurements.

Hungry4info
2010-Jan-01, 02:08 AM
Ditto! Laser pointer held down in a shoebox with tape. Laser pointer button held 'on' with tape as well.

Cut a quarter sized hole in the box and then taped foil in its place. I used a thin sewing needle to punhc two tiny holes no more than 1 mm apart and set the box about 10 feet back from the wall. Turn off the lights and enjoy.


O_O! I just set it up for myself and .... wow. Thank-you so much for that.
If I knew you in real life, I'd give you a hug.

ShinAce
2010-Jan-02, 04:47 PM
O_O! I just set it up for myself and .... wow. Thank-you so much for that.
If I knew you in real life, I'd give you a hug.

Hungry: It even works with one hole!!! The trade-off when doing this is a bigger pin hole will get more laser light through and make the pattern brighter. A smaller pin hole will give a sharper pattern, but dimmer. I noticed your profile links to extrasolar planet discoveries. I'm guessing, but this world's first picture(Gliese) of another world would have used interferometry of some extremely large ground based scopes. An easier way to cause diffraction is with a grating. Like
the venetian blinds of atomic structure, gratings are powerful diffraction tools.

Has anyone tried crystal diffraction at home? Maybe grow a salt/sugar crystal and shine light through it? I'm not sure if the wavelength of red light would be appropriate.

Stu: I just wanted to be sure PMT was a photo x-tube and not some new gizmo I should check out.

Hungry4info
2010-Jan-02, 06:41 PM
Yes, I did observe a similar diffraction pattern with a single hole. How did that happen? I figured I messed it up somehow.

ShinAce
2010-Jan-03, 03:49 PM
Yes, I did observe a similar diffraction pattern with a single hole. How did that happen? I figured I messed it up somehow.

You definitely messed it up... :p

You can think of the single pin hole as a detector of position. Since the size of the hole is about the size of one wavelength, we now know the position of the photon very well. Since we can't trick uncertainty, the momentum of the photon gets spread out into the diffraction pattern of rings. Make the hole bigger, the position is not known as well, and the pattern approaches a single point.

In classical thinking, some photon bullets would bounce off the edge of the hole and cause deflections. It's classical way to put your mind at ease, not the real mechanism.

a1call
2010-Jan-03, 07:18 PM
Any source of light would do and the original experiment was performed well before the invention of lasers. the secret is in coherence (http://en.wikipedia.org/wiki/Coherence_%28physics%29). Parallel-ness or monochromity are not required. You can turn any light source coherent including candles and flashlights by passing them through a single slit before projection onto the double slits (which happens to be the original experiment (http://micro.magnet.fsu.edu/primer/java/interference/doubleslit/) setup).
I have pointed out in the past that there seems to be a lack of detailed experimental specs in this regard on the net.

Hungry4info
2010-Jan-03, 07:22 PM
You can think of the single pin hole as a detector of position. Since the size of the hole is about the size of one wavelength, we now know the position of the photon very well. Since we can't trick uncertainty, the momentum of the photon gets spread out into the diffraction pattern of rings. Make the hole bigger, the position is not known as well, and the pattern approaches a single point.

This is pretty much what was going through my head when I messed it up. Good to hear that I did truly mess it up, I was all ready to go to the ATM and write about how I disproved all of QM :lol: