# Is interference always present in a single slit setup?

1. Sep 9, 2008

### Loren Booda

Do the walls of a single slit alone cause interference? In general terms, do all illuminated finite apparatus (e. g., a small sphere) include interference?

2. Sep 9, 2008

3. Sep 9, 2008

### peter0302

Yes. Although the pattern will be not nearly as pronounced as the double slit.

4. Sep 9, 2008

### DrChinese

It is not so much the "wall" although of course there is potentially some contribution. A single photon's ultimate path is a probability function based on all of the possible paths from the source to the target (Feynman's path integral idea). And of course, each possible path generates constructive or destructive interference relative to the others.

5. Feb 8, 2009

### vishnu korde

yes, interference takes place only at the walls and nowhere in the middle of the slit. of course if there are more number of slits the effect would be more!.... this interaction can also be seen, when sun rises above a mountain..!!... its again at the surface(mountain-air)...

6. Feb 8, 2009

### ZapperZ

Staff Emeritus
This makes no sense. If I increase the width of the slit, the "walls" are still there. Yet, the interference/diffraction effects goes weaker and in fact, will no longer noticeable at some point. This observation contradicts what you claim.

Zz.

7. Feb 8, 2009

### mn4j

This is wrong! A half slit still diffracts. Note that a half slit is part of a double slit with infinite separation. So Vishnu's point is reaffirmed.

8. Feb 8, 2009

### ZapperZ

Staff Emeritus

A diffraction pattern is WEAK to NON-NOTICEABLE as you make the slit wider. In fact, one rule-of-thumb to get a diffraction pattern is for the slit width to be comparable to the wavelength of light! You simply don't see such effects when the slit is larger. A half-slit diffraction is NOT the same as the single-slit diffraction!

And note that one can get the same Fraunhoffer interference effects not by using slits, but also by using current flow. So now, what is the "side of the slit walls" in such a case? And can you also point out to me where the slit walls actually come into the QM description of such a pattern? One sees no such inclusion in, for example, the full QM treatment of interference done by Marcella[1]

Zz.

[1] T.V. Marcella Eur. J. Phys. v.23, p.615 (2002).

Last edited: Feb 8, 2009
9. Feb 10, 2009

### mn4j

Diffraction from a half slit is noticeable. That is an experimental fact. It may be weaker but it is not unnoticeable.

10. Feb 11, 2009

### ZapperZ

Staff Emeritus
.. and explain how the half-slit diffraction and the regular diffraction are described via the effect of the slit walls! You seem to have forgotten the central issue of this thread. Even within classical picture alone, the "walls" are simply a cutoff of the light source. That's why one can do a Fourier transform of a slit using a rectangular function. It is not the source or the cause of the interference/diffraction.

Again, since this is in the QM forum, where is the function of the slit other than the restriction of the path, in the Marcella's detailed derivation of the interference effect?

Zz.

11. Feb 12, 2009

### mn4j

My understanding of the topic question was that he was asking an ontological question, of whether interference is observed in all illuminate finite apparatus, not whether the QM treatment accounts for it.

When vishnu suggested that diffraction only happens at the walls, your response to him was to suggest that he was wrong because widening the slit causes the pattern to no longer be noticeable at some point. I continue to hold that, such a claim is inaccurate because diffraction from a half slit is a well known phenomenon. Therefore no matter how large the separation between the slits, you will still observe diffraction. It is one thing to say the QM treatment does not explain this observation, it is another to claim that the observation does not exist because it does not fit the QM treatment. For this reason, questions about how the Fourier transform is done or any theoretical derivation is irrelevant.

Certainly you do not deny the fact that diffraction from a half slit is observed. So then, your claim that vishnu was wrong, is not substantiated.

12. Feb 13, 2009

### ZapperZ

Staff Emeritus
This is what I objected to:

It mentions NOTHING about half-slit diffraction, and it ignores completely the direct observation that when the slit width is larger than the wavelength, one practically loses the interference effect. If all interference is is the effect from the slit sides, then it makes no difference what size the slit width is since it is "... nowhere in the middle of the slit.."!

So you are defending that?!

And it DOES matter that the QM description makes ZERO mention of such slit-side effect. If all it is that the slit sides are nothing more than the cut-off of light path, even in the case of half-slit diffraction, then claiming that the side is the SOURCE of such phenomenon is bogus when theory does say such a thing. One can do this with the Marcella's approach, or via the classical Fresnel approach, it doesn't matter. None of them make any claim that the side of slit is anything more than simply a cut-off for the transmission of light through such a slit.

Zz.