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This question already has an answer here:

I think the title is pretty self explanatory, although I wasn't very sure about how to formulate it properly. I am currently studying Young's double slit experiment and some related stuff. One of the requirements for this experiment to work is that the light source has to be coherent.

coherent = only one wavelength and invariable phase.

So, imagine I use a normal light bulb (in stead of a light source that emits one single wavelength) and try to reproduce Young's double slit experiment. A light bulb emits multiple frequencies. The light reaching both slits will be exactly the same as it comes from the same source. So you can perfectly get positive and negative interference and see a pattern on the other side (I think). There will already some sort of interference (generating beats) of the longer wavelengths with the shorter wavelengths and so forth before they reach the screen. So this more complex waveform afterwards reaches both slits. But as this waveform is the same at both slits, I think you could still perfectly be able to see an interference pattern.

So does it really matter whether the source emits multiple more than one wavelength? If yes, why should we care?

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marked as duplicate by sammy gerbil, ptomato, Jon Custer, Bill N, heather Feb 1 '17 at 15:27

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Have a look at this :physics.stackexchange.com/q/76692 . Look at the rainbow colors of the interference coming from a non monochromatic beam. The high votes answer there is fine. $\endgroup$ – anna v Jan 29 '17 at 13:59
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The bulb's light will lose coherence after a certain distance (coherence length). After that the phase relationship is lost. So, I guess if your screen is placed at a distance which is less than the coherence length, it is fine.

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