# Did Thomas Young Misinterpret His Double Slit Experiment Results?

• B
• elou
elou
TL;DR Summary
In his famous double slit experiment Young claims that blocking one side makes the fringes disappear. I tried it with a single and a double slit, and I could block the fringes on one side one for one, while the fringes on the other side remained visible.
https://royalsocietypublishing.org/doi/pdf/10.1098/rstl.1804.0001 is the Bakerian lecture held in 1803 where Young talks of his double slit experiment. It is in fact a single paragraph in a much longer article. His article of 1801, which is often cited as the article in which said experiment is presented, is in fact a manifest in favor of the wave theory of light, and where no word is spoken of the double slit experiment.
Instead of slits, Young uses a card that cuts the beam in two halves. He claims that when he pushes a second card towards one of the sides of the center card, the fringes disappear on both sides. My own experience contradicts this claim, and I wonder if other people have tried it with similar or different results.
When the screen is far enough from the slit, the dark and bright fringes are clearly visible, and each fringe can consecutively be blocked by an obstacle put on the path of the beam, with no effect whatsoever on the remaining fringes on both sides.
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the pattern is I think not that large. Closer to 10cm

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How far are your slits from your screen?

How far from the slits are the obstacle you introduced?

You say the obstacle could block one fringe at a time. How big was it? And what was the fringe separation?

The screen, papered wall, is a little more than 2 meters, almost 3, and the obstacle less than a meter, The whole pattern is about 15cm. I did not measure any of these distances.

Then your experiment is not about blocking slits, it's just using the slits to generate a pattern and then using an object to cast a shadow on the wall. Your result is exactly as expected. You might, in principle, see some tiny diffraction effects around the edge of the shadow, but they'd be hard to see (at best).

If you want to block a slit, you need your obstacle to be in contact with the slits, not about a meter away.

Dale and Vanadium 50
elou said:
My own experience contradicts this claim, and I wonder if other people have tried it with similar or different results.

Mind you, this type of experiment is done by physics students as early as during first year labs (well, at least I did it during first year, but it's done quite early anyways). So I think you can estimate how many people has done it over the years.

russ_watters, Lord Jestocost, Vanadium 50 and 2 others
Ibix said:
Then your experiment is not about blocking slits, it's just using the slits to generate a pattern and then using an object to cast a shadow on the wall. Your result is exactly as expected. You might, in principle, see some tiny diffraction effects around the edge of the shadow, but they'd be hard to see (at best).

If you want to block a slit, you need your obstacle to be in contact with the slits, not about a meter away.
It is obvious, to me at least, that the closer the obstacle is to the slit, the more it will block the beam. Which would make the whole experiment rather meaningless. Blocking fringes only makes sense if they are visible, and the precision needed to do it close to the slit would be daunting even with modern instruments. Or am I missing something?

PeroK and berkeman
elou said:
Blocking fringes
You can block individual fringes all day long. All you are doing is casting a shadow; it will not change the interference pattern. You will only see a change in the interference pattern if you block a slit.

Dale
Ibix said:
You can block individual fringes all day long. All you are doing is casting a shadow; it will not change the interference pattern. You will only see a change in the interference pattern if you block a slit.
Is there a trustworthy link to a video recording where that was done? Not a computer animation. I have no problem imagining two different recordings, one with a double slit, the second with a single slit. But I do not understand how Young could do his experiment. I have read and reread his description and still cannot get a clear grasp of it.

I predict that this thread will undergo the same fate as the OP's other threads. As they say, IBTL.

The basic premise of the thread, that the armies of people who have done this over the last 200 years all did it wrong, and of all those people only the OP has done it right is...um...questionable.

The statement that if one obstructs one interference fringe that only one interference fringe is obstructed is not surprising, almost tautological, and in no way contradicts what Young saw.

phinds and PeroK
I predict that this thread will undergo the same fate as the OP's other threads. As they say, IBTL.

The basic premise of the thread, that the armies of people who have done this over the last 200 years all did it wrong, and of all those people only the OP has done it right is...um...questionable.

The statement that if one obstructs one interference fringe that only one interference fringe is obstructed is not surprising, almost tautological, and in no way contradicts what Young saw.
What is the difference between closing a door in one go, or very slowly? Shouldn't the end result (except noise and air displacement) be the same as far as light is concerned?

DaveE
elou said:
Is there a trustworthy link to a video recording where that was done?
Here's a photo of me bouncing a laser pointer off the millimetre markings of a ruler:

Here's a photo of the diffraction pattern

Then I shifted the laser slightly so it only illuminated the half centimeter markers - i.e., I "closed" four in five of the slits.

Even with my lousy left-handed cell phone photography of an experiment jury rigged out of parts to hand you can see the pattern has changed. You can do the experiment yourself and watch the pattern change as you adjust the aim point of the laser and "close the slits".

sophiecentaur, berkeman and Dale
elou said:
What is the difference between closing a door in one go, or very slowly? Shouldn't the end result (except noise and air displacement) be the same as far as light is concerned?
In this case it is the difference between doing the experiment you are trying to replicate and not doing it. Again, in this experiment you block the slit, not the fringes.

Dale said:
In this case it is the difference between doing the experiment you are trying to replicate and not doing it. Again, in this experiment you block the slit, not the fringes.
Okay. Could you explain to me please, how Young did it? Not how it could be done now.

elou said:
Okay. Could you explain to me please, how Young did it? Not how it could be done now.
As far as I know Young only did two slits. I do not think that Young blocked a slit. But I am not a history buff, so I could easily be mistaken. Doesn’t the reference you posted say exactly what he did?

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elou said:
Okay. Could you explain to me please, how Young did it? Not how it could be done now.
No one knows more about how Young did it than Young, and you’ve already linked his monograph.

How am I supposed to understand this. other than that fringes can be extinguished separately, one after the other?
"When the interposed screen was more remote from the narrow card, it was necessary to plunge it more deeply into the shadow, in order to extinguish the parallel lines;"

Dale said:
Doesn’t the reference you posted say exactly what he did?
Yes it does, And I have trouble figuring it out exactly, because I see no difference with what I did.

How would men nowadays close a slit, starting with two?

elou said:
"When the interposed screen was more remote from the narrow card, it was necessary to plunge it more deeply into the shadow, in order to extinguish the parallel lines;"
I think Young is just casting a shadow with the second card, so he demonstrates that the fringes are coming from the diffracting object rather than being some surface effect in the screen.

Ibix said:
I think Young is just casting a shadow with the second card, so he demonstrates that the fringes are coming from the diffracting object rather than being some surface effect in the screen.
I can't find any clue that this was a concern for him. Can you? It still leaves the question of "extinguishing fringes" open.

elou said:
I can't find any clue that this was a concern for him. Can you? It still leaves the question of "extinguishing fringes" open.
No it doesn't. This is 2024, not 1803. What Young did is of only historical interest now. Nothing in modern physics hinges on what Young did or did not do 220 years ago.

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russ_watters, Dale, weirdoguy and 1 other person
PeroK said:
No it doesn't. This is 2024, not 1803. What Young did is of only historical interest now. Nothing in modern physics hinges on what Young did or did not do 220 years ago.
Dou you have a 2024 explanation different from Young's?

elou said:
Dou you have a 2024 explanation different from Young's?
Well, a little thing called Quantum Mechanics was developed in the 20th Century. And, as other posters have said, anyone taking an undergraduate course in physics (or even an advanced high-school class) will have done these experiments themselves with modern equipment.

So, yes, physics has indeed progressed in the past 220 years!

russ_watters
Ok, here's what you do.

You get a laser and shine it through something that will cause the beam to diverge - e.g. a pair of glasses for short sight. Then you bend a paperclip so that it can pass through the beam and place it immediately after the lens.

The diffraction pattern in the fairly near field looks like this:

The camera is rotated at an awkward angle for that shot - horizontal in reality is about 45° up to the right. You can just see fringes in the middle of it. They're much more obvious in reality, but even with the heavy post-processing I did on that they're very faint on camera. It's all washed out by the bright blobs.

Set up your screen maybe a meter from the clip (the correct distance will probably depend on your lens choice). Play around until you see fringes.

Note that the paper clip is horizontal. Get a piece of card and hold its edge parallel to the paper clip, and close to the clip - an inch or two away. Lift or lower it until you see it casting a shadow on your screen. When it starts to obscure the upper or lower bright blob you'll see the fringes between the blobs start to flow and eventually disappear.

Don't touch the table where your stuff is mounted while you're doing this or you'll wash the fringes out from vibration.

Again, this is a one-man job all knocked together in half an hour from parts to hand in my home (yes, those are my glasses, which made the rest of this a bit of a challenge, and the laser pointer button is held down by a piece of lego, a book, and my son's piggy bank). You could do a much more professional job with fairly minimal investment.

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Nugatory and Dale
Maybe you will remember that I used a laser and dia's with single and double slits to create an interference pattern. So, I don't really understand what you are trying to prove. That certainly does not help with explaining, in modern terms, how closing a slit at once is different from closing it slowly. And how Young's explanation has been superseded by QM. Mind you, I am not saying it has not, but it would be interesting to know what that explanation is exactly.
I also have closed a slit, gradually. Until only one small dot, hardly visible, remained. So, the interference pattern did disappear, one fringe at a time.

edit: I have never seen a modern double slit experiment whereby the interference pattern did disappear because of something the experimenter did, the way Young describes his experiment. I have seen plenty of computer simulations though.

elou said:
So, I don't really understand what you are trying to prove.
Is that directed at me? You asked what Young was doing and how to repeat it. I did it and wrote it up.
elou said:
That certainly does not help with explaining, in modern terms, how closing a slit at once is different from closing it slowly.
It isn't. Except on the sub-nanosecond time scale, I suppose.

Ibix said:
Is that directed at me? You asked what Young was doing and how to repeat it. I did it and wrote it up.

It isn't. Except on the sub-nanosecond time scale, I suppose.
It is not a matter of creating an interference pattern. It is trying to find out how Young's explanations make sense, and whether modern explanations are better.
Like I said, I do not understand Young's explanations. I cannot see how he could come to his conclusions.

edit: btw, my reaction was to all the replies that came afterwards, not specifically yours. I should have replied to each one separately.

elou said:
Like I said, I do not understand Young's explanations. I cannot see how he could come to his conclusions.
You said that 27 posts into the thread. Before that you were bemoaning not being able to understand how Young could do his experiment:
elou said:
But I do not understand how Young could do his experiment.
I've told you how to do that. The key difference between Young's set up and yours is the diverging nature of Young's natural illumination versus a collimated laser beam (hence the use of my glasses to make my beam diverge). Young was working in the near field (which is how he gets "shadow with fringes in it" rather than a ##\mathrm{sinc}^2## intensity profile), while almost certainly anything you are doing is in the far field.

As to his explanation, what's he saying that's more complex than "we get diffraction, and when I modify the slit I get different diffraction"?

Ibix said:
You said that 27 posts into the thread. Before that you were bemoaning not being able to understand how Young could do his experiment:

I've told you how to do that. The key difference between Young's set up and yours is the diverging nature of Young's natural illumination versus a collimated laser beam (hence the use of my glasses to make my beam diverge). Young was working in the near field (which is how he gets "shadow with fringes in it" rather than a ##\mathrm{sinc}^2## intensity profile), while almost certainly anything you are doing is in the far field.

As to his explanation, what's he saying that's more complex than "we get diffraction, and when I modify the slit I get different diffraction"?
"I do not understand how Young did his experiment" and " I do not understand Young's explanations" are one and the same thing. He says that the interference pattern disappears, and I wonder: from where? From the wall, from the small screens close to the slip, the ones further away?
And it is much more that "a different diffraction". For him, the disappearance of the interference pattern did not have the same meaning as it did since the 20th century. It was simply a confirmation of the wave nature of light. But his explanation of the disappearance of the interference pattern does not make sense, whatever the nature of light is. Not if it turns out that the disappearance can be gradual instead of sudden. And not if it turns out that only the fringes blocked are involved, and not automatically both sides.

elou said:
He says that the interference pattern disappears, and I wonder: from where?
From the screen, where he was observing it.
elou said:
For him, the disappearance of the interference pattern did not have the same meaning as it did since the 20th century. It was simply a confirmation of the wave nature of light.
What do you think the meaning of it is in the 21st century if not a confirmation of the wave nature of light?
elou said:
But his explanation of the disappearance of the interference pattern does not make sense, whatever the nature of light is.
He adjusts the things that are causing the diffraction and the pattern changes. How are you going to explain that without waves? Bear in mind that you can see interference fringes, which are a strong hint that you've got waves anyway.

If you really want to understand it you need the Fresnel-Kirchoff integral, probably via Huygens' Principle first. I'd recommend buying Born and Wolf's Principles of Optics rather than randomly attempting to replicate two century old experiments.

russ_watters
Thank you for your advice. This thread can be closed.

Ibix said:
You said that 27 posts into the thread.
The ground is certainly shifting. It started out with the claim Young (and the thousands, possibly millions of people after him) measured it wrong. Now it's that his explanation is wrong.

This ground was covered in his previously closed thread. While I suppose we can go around and around again, I very much doubt much will have changed in the intervening two weeks.

Ibix said:
If you really want to understand it you need the Fresnel-Kirchoff integral, probably via Huygens' Principle first. I'd recommend buying Born and Wolf's Principles of Optics rather than randomly attempting to replicate two century old experiments.
elou said:
Thank you for your advice. This thread can be closed.

This ground was covered in his previously closed thread. While I suppose we can go around and around again, I very much doubt much will have changed in the intervening two weeks.
Presumably @elou will take the advice from both threads and read some textbooks before posting any more questions like this.

This thread is now closed. Thanks to all who have been trying to help the OP understand this material better.

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