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I've read in many places that it's to do with illuminating both double slits by diffraction, even though the interference pattern on the screen would be an overlap of different interference patterns.

However, would the light that emerges from this wide single slit still be coherent? That is, has a constant phase difference, assuming that it's light from a filament lamp that has just passed a filter.

I thought that it wouldn't be coherent, since light can pass through it coming in all different directions, whereas a narrow single slit confines the waves into an almost perfect narrow path to provide a constant phase difference. Is this right?

XXb8
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  • Duplicate - Why must the single slit in a double slit experiment be narrow? – Farcher Feb 16 '20 at 19:05
  • If the single slit is wide, mutually displaced interference patterns will be produced and, if the slit is really wide the bright fringes of one pattern will fill the dark fringes of another, and so on, so no pattern will be visible. So to be a coherent source the slit must be very narrow. The light it sends out must also be a good approximation to monochromatic. – Philip Wood Feb 16 '20 at 19:27
  • @PhilipWood so it's to do with light waves from different directions coming through the single slit rather than anything to do with diffraction, and then all these different direction waves forming their own displaced ineterference pattern? – XXb8 Feb 16 '20 at 19:32
  • The single slit does have to be narrow enough for waves from it to diffract enough to reach both slits of the double slit. But why have a single slit at all? wouldn't light from (say) a filament lamp reach both slits anyway? Yes , but then you have the incoherence problem that I talked about in my previous comment. I'm not sure, incidentally that this problem is "to do with light waves from different directions coming through the single slit"; I'd say that it's more to do with the slit not approximating to a point (or line) source. – Philip Wood Feb 16 '20 at 22:33
  • @PhilipWood and how would mutually displaced interference patterns be produced? – XXb8 Feb 16 '20 at 23:18
  • A wide slit (or an extended light source) can be thought of as a collection of narrow slits. Each of these will produce its own fringe pattern. The patterns won't co-incide, because at any given point on the screen, the path differences for light from the various narrow slits will be different. – Philip Wood Feb 17 '20 at 08:03
  • @PhilipWood but wouldn't there be superposition of waves from these different narrow slits before they pass through the double slits? So creating an interference pattern that then passes through the double slits? – XXb8 Feb 17 '20 at 08:49
  • The narrow slits aren't in phase with each other, nor is there a constant phase relationship between them, so you don't get interference between light from different narrow slits for the same reason that you don't get interference between sound from two violins, even if they are playing what to our ears is the same note (though you might get interference between sound from two flutes - producing more or less continuous wave-trains). – Philip Wood Feb 17 '20 at 10:05

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