Double slit question - how far apart can the slits be?

[brajesh]

TL;DR

I was wondering how far apart the slits can be and the double slit experiment still works?

How far apart can the slits be and the double slit experiment still works i.e the wave interference pattern is seen on the target screen?

 

[PeroK]

Summary:: I was wondering how far apart the slits can be and the double slit experiment still works?

How far apart can the slits be and the double slit experiment still works i.e the wave interference pattern is seen on the target screen?

It depends on the spread of the incident beam, which needs to cover both slits. If you put the slits too far apart, then all of the incident beam is absorbed by the first barrier.

 

[brajesh]

So does that mean that theoretically, let's say the incident beam spread is an arc of 30 degrees, then if I move the slits apart AND also keep moving the barrier further away such that both slits are still covered by the beam, that I could have a very large separation and still continue to see the effect?

 

[PeroK]

So does that mean that theoretically, let's say the incident beam spread is an arc of 30 degrees, then if I move the slits apart AND also keep moving the barrier further away such that both slits are still covered by the beam, that I could have a very large separation and still continue to see the effect?

The double-slit is not anything special: it's just the way nature works. It's not a trick. Whatever set up you have, you can do the calculations and predict the result.

To see an interference pattern you need:

1) The beam must straddle both slits (approximately equally).

2) The slits must be narrow enough for significant single-slit diffraction.

3) The slits must be close enough together so that the single-slit diffraction patterns significantly overlap.

There's no magic about this. If the slits are too wide, you don't get enough diffraction; and, if they are too far apart you get two separate single-slit patterns.

 

[Nugatory]

So does that mean that theoretically, let's say the incident beam spread is an arc of 30 degrees, then if I move the slits apart AND also keep moving the barrier further away such that both slits are still covered by the beam, that I could have a very large separation and still continue to see the effect?

Yes, although as we move the barrier away the intensity goes down so there's less of anything to see - or equivalently the dots per second rate goes down and it takes longer for the pattern to build up.

 

[DrChinese]

So does that mean that theoretically, let's say the incident beam spread is an arc of 30 degrees, then if I move the slits apart AND also keep moving the barrier further away such that both slits are still covered by the beam, that I could have a very large separation and still continue to see the effect?

What would you be trying to accomplish by having the source 30 degrees (or whatever angle) off the vertical? Normally you want the source to go straight to the slits so that they are illuminated as close to equally as possible. To the extent they are not equal, the interference effect is lessened.

You can make the distance between the slits any value you like. Normally it is recommended to have the slits about a wavelength apart, and on the order of a wavelength wide, but that is just a starting point. There are a lot of ways to create interference effects, and in all of them the key is that there are (at least) 2 ways for light to get to a spot. The less distinguishable the path, the greater the interference effect (which obviously can be positive or negative).

 

[hutchphd]

It depends on the spread of the incident beam

Let me try to be semantically pedantic. I would prefer to talk about the "width" of the essentially collimated incident beam. "Spread" implies to me a large lack of incident collimation and simply complicates the picture.

 

[PeroK]

Let me try to be semantically pedantic. I would prefer to talk about the "width" of the essentially collimated incident beam. "Spread" implies to me a large lack of incident collimation and simply complicates the picture.

I was using "spread" as a synonym for statistical dispersion:

https://en.wikipedia.org/wiki/Statistical_dispersion

 

[hutchphd]

Oh my apologies...I didn't look carefully enough. I should never try pedantry.