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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

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