# Can interference occur without the use of single slit?

1. Jan 23, 2015

### ovais

I wonder can a sustained interference pattern be obtained in Young double slit experiment if we do not use single slit between source of light and the double slits when one has a monochromatic source in the experiment.

Regards

2. Jan 23, 2015

### Staff: Mentor

You need a source of coherent light. The single slit is by far the cheapest, easiest, and lowest-tech coherent light source, but any coherent light source will do.

3. Jan 23, 2015

### ovais

First of all thank you for giving your precious time for the reply.
Say the only information provided to me is that my source is monochromatic, the questioner was asking me if you do have a monochromatic source and two slits after it, will you get any sustained interference pattern on the screen placed after some distance.

4. Jan 23, 2015

### ovais

It appears as he was asking that Could a pure monochromatic source amount to a coherent source?

5. Jan 24, 2015

### my2cts

Interference always occurs but to make it visible you need a coherent source.
If the source the result is a superposition of interference patterns in which the intensity variation is erased.

6. Jan 24, 2015

### ovais

Can a(single) pure monochromatic source(without single slit) act as coherent source?

7. Jan 24, 2015

### my2cts

Such a source lacks spatial coherence and can be seen as a superposition of spatially coherent sources (think of your source as consisting of adjacent slits). You will observe the interference patterns of each of these slits superimposed on each other. This will erase the characteristics of the individual interference patterns

8. Jan 24, 2015

### phinds

Well I guess it COULD, but there is no reason to assume that it does in any particular case. Coherent is more restrictive than monochromatic. Do you understand the definitions of the terms?

9. Jan 24, 2015

### blue_leaf77

Monochromaticity ensures perfect temporal coherence, but not always with spatial coherence. For example you have a light bulb (extended source) containing one species of atom, and further assume these atoms magically emits one exact frequency from $t = -\infty$ (perfect temporal coherence). These atoms do not,in general, emit radiation all in phase, hence the spatial coherence is limited. In this case you still need a pinhole.

10. Jan 24, 2015

### my2cts

For interference you need both temporal (monochromaticity) and spatial (collimation) coherence.
If you use an imaging system you can do without temporal coherence.

11. Jan 24, 2015

### blue_leaf77

So whether one can maintain the interference pattern depends on the type of the source. If this source is of one single atom, then yes one doesn't need the first pin hole at all.

12. Jan 24, 2015

### ovais

13. Jan 24, 2015

### ovais

Will sustained interference pattern observe in this source is assumed to be perfectly monochromatic

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14. Jan 24, 2015

### ovais

Yes phinds I know their definitions the confusion I am having(which is very well being getting clear through you guys lies in to understand whether a(single) monochromatic source be turned into a coherent source any how?
Like what I shown in the figure.

Last edited: Jan 24, 2015
15. Jan 24, 2015

### ovais

Blue_Leaf77 your. statement, Monocromocity ensures temporal coherence but not always spatial coherence, is very informative. But I will be very thankful to you if tell me it in more simpler way of my level.

As per the assumptions you also took that let us suppose that the extend source somehow(magically) produces only single sharp wavelength, then how do them(atoms), in general will not emit waves all in phase, causing limited spatial coherence.

If you tell this in more descriptive way I will be very thankful to you I am sorry for my very little basic knowledge.

The same think can be answered if you just tell me if their will be sustained interference in the case of which a picture I sent with a little backup of knowledge.

Regards

16. Jan 24, 2015

### my2cts

17. Jan 24, 2015

### ovais

I read your statement and see it very seriously. But their is some difference I noticed between what I asked at first in my question which you clearly answer in that very reply and the situation(something different with one source please do look picture you will get an idea what different thing now I want to look at) which is in the picture now.

As the problem of not having spatial coherence is appear to be absent in this case of picture. I beg you to reply in that case in more open way for I don't hesitate to beg for knowledge as knowledge is the only thing worth begging.

Regards

18. Jan 24, 2015

### blue_leaf77

Monochromatic light only has one wavelength, only contains one harmonic, in math form you will write monochromatic light as $E_0 \cos{\omega t}$. Such wave when compared to its copy will have fixed phase relation from $-\infty$ all the way up to $+\infty$, that is $\Delta \Phi \neq f(t)$. This means this wave has infinite coherence length which is the definition of perfect temporal coherence. I hope this link can help you further: http://www.rp-photonics.com/coherence.html

19. Jan 24, 2015

### my2cts

Your picture has no information on the spatial coherence of the source.

20. Jan 24, 2015

### Staff: Mentor

Yes. The light emitted from each slit will interfere and form an interference pattern on the screen.

21. Jan 24, 2015

### my2cts

I disagree, anything is possible. It completely depends on how each of the openings is illuminated. The picture contains no information on this.
If both openings are illuminated from the same point source an interference pattern will form, namely a coherent sum of the individual slit diffraction patterns. Note that the two slits are then each illuminated from a different direction. If the source is spatially extended the interference pattern is the sum of all such patterns, one per source point. This sum is incoherent if the source points are incoherent. The more extended the source the less of an interference pattern will remain. If all points of the source are coherent, some complex interference pattern will result.

22. Jan 24, 2015

### Staff: Mentor

How can you get incoherent source points if the source itself is given as a monochromatic source?

23. Jan 24, 2015

### my2cts

A monochromatic source is temporally coherent but not necessarily spatially coherent.
For the latter, light from different directions also has to have a phase relation.
If monochromatic light comes from a diffuse scatterer, such a phase relation is lost.

24. Jan 24, 2015

### ovais

Here are the things hooking up. I fail to get a common answer, people even in my Institute are telling different answers without giving a descriptive answer for their "yes" and "no" and I have my own thoughts on "yes" and "no" for the above question. Which if cleared sufficed the purpose of the thread. And I am here just to clear the doubt and I feel as opposed to philosophy where disagreements at a subject are allowed, physics is an exact science, answers with disagreements needs to consider again.

Regards

25. Jan 24, 2015

### Staff: Mentor

And how does this tie into the example given by the OP?