Double Slit Experiment

[NovaJoe]

Well hello Astronomy Cast,
My name is Joseph Stayton (stay-ton) and I am from and live in Tacoma Washington. In the Astronomy Cast episode titled “wave-particle duality,” I couldn’t help but notice that the description of the double slit experiment is described in two distinct manors, which are inconsistent. In one, the experiment is described in the same context that I have always understood. In another, Pamela describes the effect observed when a very precise light source is projected through a slit (through one slit only) producing an interference pattern from which a different interference pattern is observed when the same light is projected through two slits. She also agrees with Frasier when he states that two slits are needed for an interference pattern to be observed i.e. with only one slit there is no interference pattern observed. Pamela also describes a “diffraction pattern” being observed with only one slit, which I interpreted as another word for interference suggesting an interference pattern. Prior to listening to this pod cast, every description of the results observed from the double slit experiment that I have encountered describes interference patterns as being observed only when there are two or more slits. Pamela describes how anyone can set up this experiment at home by very precisely setting up four razorblades to construct two slits. If you have set them up properly, when a laser is projected through the latter slits an interference pattern will be observed. Now if one of the two slits is blocked the interference pattern, according to my understanding prior to this episode of astronomy cast, should disappear.
Because Pamela describes in one instance that an interference pattern will be observed when a laser is projected through one slit, which is in opposition to what I have come to understand of the double slit experiment , me and a friend went out and got a couple of lasers, some razor blades and set up the experiment. I was expecting to observe an interference pattern when we projected a laser through two slits but not when projected through only one. Nevertheless, no matter what adjustment we made to try and make sure that we only allowed light through one slit we always observed an interference pattern, and in fact when we examined the laser beam itself, we observed an interference pattern by simply pointing the laser at the wall without going through any slits at all, though it was noticeably different than either of the other interference patters we observed, it was still there. We tried all three instances with several different lasers set up in different apparatus’s and in all instances we observed interference patterns, however different they may have been. The only way we were able to observe the lasers on the wall without producing interference patterns is when we placed both the laser and the slits very close to the wall in comparison with the distances required for us to observe these interference patterns. Would it possible that the lasers we used are somehow causing these interference patterns, possibly because they were cheap and therefore not as precise of a light source needed to conduct this experiment? Also, are there other instances when this experiment has been conducted in which the experimenters observed similar results, if not, was it just an honest mistake when Pamela describes an interference pattern being observed when light is projected through only one slit? Pamela describing interference patterns being observed when light is projected through one slit caused some confusion, and that’s what motivated me and my friend to conduct this experiment ourselves, the results of which has further confused us. Is there any way you guys could possibly clarify this confusion?

Appreciatively,
Joseph A. Stayton

[Ken G]

Pamela also describes a “diffraction pattern” being observed with only one slit, which I interpreted as another word for interference suggesting an interference pattern.

Diffraction is a ramification of interference, but when some people refer to an "interference pattern" they mean more than that-- they mean interference between two separated sources (the slits) that can produce really obvious bright and dark bands. The words aren't very clear-- sometimes we call something a "diffraction pattern" even when it shows bands of bright and dark, and an interference pattern even when it doesn't. So we always have to look at the larger context.

I was expecting to observe an interference pattern when we projected a laser through two slits but not when projected through only one.

It's pure semantics, the predictions are clear enough. A single slit can produce a pattern of bright and dark bands, that we would be completely justified in calling an interference pattern. It's just that the bands get more obvious with two slits, so there's a kind of lazy convention of calling the two-slit pattern "interference" and the one-slit pattern "diffraction". There is actually no fundamental distinction there, it just adds to the mystique of two slits versus one. The bottom line is, this is all wave mechanics-- waves on water do all the same things with one opening or two.

Nevertheless, no matter what adjustment we made to try and make sure that we only allowed light through one slit we always observed an interference pattern, and in fact when we examined the laser beam itself, we observed an interference pattern by simply pointing the laser at the wall without going through any slits at all, though it was noticeably different than either of the other interference patters we observed, it was still there.

Yes, that is because all light propagation involves interference. In that sense, everything light does, including reflection off a mirror, is an "interference pattern". You are talking about bright and dark bands, which you can get with one opening-- it's just not as typical or as obvious.

Would it possible that the lasers we used are somehow causing these interference patterns, possibly because they were cheap and therefore not as precise of a light source needed to conduct this experiment?

Yes, the bandwidth, or precision, of the laser is an issue. It gets a little complicated, but if you research "single slit diffraction pattern", you can probably find the details you need to understand what you are seeing. But the bottom line is, your confusion stems from the imprecise language that gets used around interference and diffraction, not because there's anything fundamentally wrong.

[alainprice]

There are 2 different forces at work here.

The first is uncertainty, the second is wave-like interference.

When we have a single slit, the width of the slit might be less than the particle wavelength. If this condition is satisfied, we know very well the position of the particle when it passes through the slit. Uncertainty plays a trick by dispersing the momentum of the particle as we narrow down a position. Think of this when you see a single slit. This is a 'diffraction' pattern due to uncertainty.

When we have two slits, the particle can go through either slit equally, and the result is an interference pattern between the two slits. This is a property of the wavelength of the particle and the distances between the 'narrow' slits. Here we have a real 'interference' pattern.

I hope that helps.

p.s. I have a red laser pointer and would like to do the experiment myself. I can calculate the distance of fringe maxima and minima based on the setup, but I can't physically make the slits. What material should I use for the sheet and how should one make the slits?

[dhd40]

A single slit can produce a pattern of bright and dark bands, that we would be completely justified in calling an interference pattern.

There is an easy way to prove this. Hold your palm against a light background (the daylight sky, the screen you´re just now looking at). Now move your head towards the palm, close one eye, and see what happens between your fingers: a beautiful black-and-white pattern

Is this interference, or diffraction, or... ?

NovaJoe, please try it. Hopefully, my description of this "experiment is helpful.

[Ken G]

There are 2 different forces at work here.

The first is uncertainty, the second is wave-like interference.

Those are indeed at work, but the idea that these are two separate things is, I think, the main source of the confusion here. There is some value in seeing those distinctions, but uncertainty and interference are just two halves of the same coin, it's all "wave mechanics", and both are in play any time you have waves. The idea that one applies to a single slit and the other to a double slit is the error that the OP is asking about-- both are fully at play in both situations, so there is no fundamental distinction between a "diffraction pattern" and an "interference pattern". However, in the specific context of two slits, people tend to reserve the latter term for the clear bright and dark variations in the two-slit pattern, but the distinction shouldn't be taken too literally.

[a1call]

and in fact when we examined the laser beam itself, we observed an interference pattern by simply pointing the laser at the wall without going through any slits at all, though it was noticeably different than either of the other interference patters we observed, it was still there.

Are you certain that some of the "patterns" you observed where not the Speckle phenomena?

[alainprice]

Those are indeed at work, but the idea that these are two separate things is, I think, the main source of the confusion here. There is some value in seeing those distinctions, but uncertainty and interference are just two halves of the same coin, it's all "wave mechanics", and both are in play any time you have waves. The idea that one applies to a single slit and the other to a double slit is the error that the OP is asking about-- both are fully at play in both situations, so there is no fundamental distinction between a "diffraction pattern" and an "interference pattern". However, in the specific context of two slits, people tend to reserve the latter term for the clear bright and dark variations in the two-slit pattern, but the distinction shouldn't be taken too literally.

Yeah, one thing I forgot to mention is that the double slit pattern is really just the interference of 2 diffraction patterns. Diffraction is always working hard.

[Ken G]

Yeah, one thing I forgot to mention is that the double slit pattern is really just the interference of 2 diffraction patterns.

Right, and diffraction is interference too. Or more correctly, diffraction stems from an interruption in interference-- the interruption generated by the the wall where the slit isn't.

[alainprice]

Bump.

a1call: I believe he described all 3 situations. First, a single diffraction pattern. Secondly, a double slit interference pattern. Lastly, speckle.

I just tried this 15 minutes ago with a piece of aluminum foil, a needle, and a 1$ laser pointer. You gotta love lasers.

With 2 pinholes less than 1 mm apart, I observed the familiar double-slit pattern. Even though I was using 2 point sources for the interference, I had clear light and dark bands. As expected, they were perfectly spaced and parallel.

I then tried it with a single hole, and got an artichoke looking pattern. I did have a decent sized bright spot in the center and a single ring; it looked like Saturn. The spot was roughly 1/2" with the pinhole 10 feet away. Obviously, the hole wasn't perfect nor is the consistency of the foil thickness. It was a cool pattern to see, nonetheless.

[alainprice]

This thread needs pics, so I repeated the single pinhole diffraction experiment today and took some pictures. It came out better than the other day, but it still isn't nearly perfect.

I will repeat the double pinhole some other day. The straight fringes(hyperbolic actually) come out great no matter how sloppy you are making the pinholes.

[HenrikOlsen]

What happens is interference between the edge diffractions.

Very nice picture BTW.

[alainprice]

Thank you for the encouragement, Henrik.

Here are some pics of a double pinhole interference pattern. The first is a full field view of the pattern. The second is a close-up of the center with lower exposure. The last is what happens when you widen the holes too much. The smaller the hole the better. I'm using a Canon S3IS and 1st pic is 15 second exposure at F2.7. The second is roughly a 3" exposure. The last one is another 15 second exposure.

To the naked eye it seems the separation between the holes nearly matches the hole size. This might explain the fact that it looks like a 2 source interference pattern mated to a single source diffraction pattern. The last time I tried this the underlying single hole diffraction(circular rings) wasn't nearly as obvious.