For incoherent light sources, they are passed through a single slit first. The interference pattern of the single slit then modulates the intensity of the double slits' interference pattern.

But what about lasers? They don't need to be passed through a single slit first, so would their intensity curve be modulated by a single slit 'envelope'?

Here is the graph we expect if passed through a single slit only (right), and a single followed by double slits (left) enter image description here

  • $\begingroup$ Lasers are often assumed to have a Gaussian beam profile, not the abrupt cutoff of a slit or a circular aperture. So the profile in the far field is not an Airy pattern. $\endgroup$ – Pieter Feb 17 at 23:36
  • $\begingroup$ @Pieter so neither of these graphs would show the intensity distribution of a laser when passed through double slits? $\endgroup$ – XXb8 Feb 18 at 8:54
  • $\begingroup$ The figure on the right shows a double-slit interference pattern with the envelope given by the width of those slits. It does not need a laser, just a point source. You can see it on your retina when looking at an LED through a double slit just in front of your pupil. $\endgroup$ – Pieter Feb 18 at 9:03
  • $\begingroup$ @Pieter So the intensity graph for any point source (including a laser) would still be the graph on the right? I don't understand this, since if they were already coherent, they weren't passed through a single slit, so where would the 'envelope' come from? $\endgroup$ – XXb8 Feb 18 at 9:06
  • $\begingroup$ @Pieter I think my confusion comes from the fact I don't know where the single slit envelope comes from. I assumed it came from the single slit we first pass incoherent light through first to make it coherent. This single slit then acts as an envelope for the two double slits. Is this correct? $\endgroup$ – XXb8 Feb 18 at 9:16


The left hand graph is the intensity pattern for a single slit all by itself.

The right hand graph is the intensity pattern for a two parallel slit arrangement with the two slits each with the same width as that of the single slit which produced the left hand intensity pattern.
The double slit intensity pattern is an equally spaced double slit interference pattern modulated in intensity by a single slit diffraction pattern.

The effect of changing the width of the slits is described in this answer Young's double slit experiment, the slit width.

When using a laser the additional single slit is not needed because it provides light with a single coherent wavefront whereas that is not so with other light sources as described in the answers to the question Why must the single slit in a double slit experiment be narrow?

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  • $\begingroup$ I've edited the photo description to clarify this -thank you. But the question was whether a laser passed through two double slits would form the same modulated intensity pattern for its interference waves as the graph on the right, since it was never passed through a single slit in the first place so there would be no modulating 'envelope'? $\endgroup$ – XXb8 Feb 17 at 23:18
  • $\begingroup$ @XXb8 Yes and that is what I tried to explain. $\endgroup$ – Farcher Feb 17 at 23:58
  • $\begingroup$ ah, yes sorry! So for a laser, it would still look like the far right graph? $\endgroup$ – XXb8 Feb 18 at 8:52
  • $\begingroup$ Here is a video which shows the experimental setup of laser, double slit and screen and no single slit producing the interference pattern. $\endgroup$ – Farcher Feb 18 at 13:39

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