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Thomas Young used a single slit between the light source and the double slits. I can't understand why did he used the single slit, since the light from only one source is coherent already or isn't it? Does the narrow single slit make incoherent source coherent?

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"since the light from only one source is coherent already" Most day-to-day sources (sun light, incandescent lights, etc...) are incoherent. – dmckee Nov 1 '13 at 23:01

At the time of Young there were no lasers to provide coherent light. Incandescent light is incoherent because it comes from many sources and the same is true for sunlight. By passing the light through the one slit

young's slit

he created a single coherent wave front . It is worth reading his description "on the nature of light and colors" in the link above.

Edit: I have not addressed the "why" a slit induces coherence. It has to do with wavelength : the size of the slit must be such that, similar to a diffraction grating, it diffracts specific wavelengths differently and also acts as a point source creating a coherent wavefrong. Here is an illustrated description of coherence. Here is a blog entry discussing coherence from thermodynamic sources .

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Doesn't the single slit act more like a prism, selecting certain frequencies of light? Coherent light must all be in phase and the single slit doesn't do that. – Brandon Enright Apr 30 '13 at 20:28
    
@BrandonEnright the grating also separates wavelengths. thus your question is back to why a single slit creates coherence – anna v May 1 '13 at 3:35
    
Yeah but a prism separates wavelengths too. My point is that even if the single slit only allows a certain color (wavelength of light) to pass, isn't the phase still random? That is, it's monochromatic and mostly collimated but not coherent. – Brandon Enright May 1 '13 at 6:41
    
@BrandonEnright read the links in my edit. It seems there exists a coherence length for thermodynamically produced photons too. The incandescence manifests in small coherent regions even when the whole is incoherent. a slit lets many wavelengths through, it has to be of the order of the coherence length . – anna v May 1 '13 at 7:19
    
It's important here to remember about the difference between temporal and spatial coherence. Temporal coherence is the one directly related to different wavelengths and how monochrome the light is. If I'm not mistaken in putting it like this, temporal coherence is about relative phase difference. Spatial coherence, on the other hand, which is related more to light of the same wavelength, is about the relative placement in space of the phases themselves, that is, to what degree the light travels crest-to-crest/trough-to-trough. (Cont.) – David Reishi May 12 at 16:34

"Thomas Young used a single slit between the light source and the double slits. I can't understand why did he used the single slit, since the light from only one source is coherent already or isn't it? Does the narrow single slit make incoherent source coherent?"

This answer is not to imply that Anna V's is incorrect.

One of the sources of confusion when learning about the double-slit experiment is that, sometimes you'll see a setup with the light-source directly facing the double-slit; whereas sometimes you'll see a setup with a single-slit intervening between the light-source and the double-slit. This might be what prompted the OP to ask the question.

If so, then the question should be answered by saying that some light-sources emit light that's spatially coherent enough to see interference, whereas some light-sources don't. If we leave lasers out of the discussion for a moment, then we can say that it's no coincidence that the closer and closer a light-source is to being a point-source, the more spatially coherent the light emitted from it, and hence the less one needs to intervene a single-slit between the source and the double-slit. For the point-source, emitting light in theory from a single point, and the single-slit, which could just as well be a single-pinhole, carry out essentially the same role in terms of making the light spatially coherent. So that, yes (to answer your second question), the single narrow slit makes spatially incoherent light coherent.

About lasers, they work differently in that, to put it most simply, they're not "point-sources" but emit spatially coherent light. Needless to say, you'd never see a double-slit setup using a laser and an intervening single-slit.

BONUS! - I'm going to end this answer with a further question for the OP or anyone else who's interested. If a small slit makes spatially incoherent light coherent, then why does the double-slit experiment require a point-source or an intervening single-slit at all? Shouldn't the two slits comprising the double-slit itself be all that's needed to make the light coherent enough to interfere? (Hint: tracing the light back from the interference to the source may help.)

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