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I have seen that when we use a spectrometer for performing an Optics experiment involving a single slit or double slit to study , say the Fraunhofer Diffraction pattern or some interference pattern, we use Schuster's method to focus the fringe pattern on the focal plane of the eyepiece of the telescope. But, I can see the fringe pattern (just that it has formed, not with such high resolution like the telescope) by naked eye through the narrow slit(s), also.

So my question is:

Where does the interference pattern or diffraction pattern due to a single or double slit placed in front of a light source form, in this case?

Does it form at infinity, as the theory says; or at some other place? Like the diffracting edges of the narrow slit? Or the focal plane of the eyepiece of telescope? Or does it vary depending on how I look at it?

I couldn't find a proper answer anywhere.

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    $\begingroup$ The interference pattern is expressed all along the propagating wavefunction. This is most clearly seen in, e.g., water wave interference near coastlines. $\endgroup$
    – Jon Custer
    Nov 6, 2017 at 16:29
  • $\begingroup$ @JonCuster The propagating waves interfere all through space, between the slits and a possible screen. If what you say is true, then why dont we see interference patterns everywhere in space? $\endgroup$
    – SchrodingersCat
    Nov 6, 2017 at 18:01
  • $\begingroup$ You are right about the edges. Any fringe pattern can be derived on a particle (photon) bases from the edges to the screen. The process begins at the edges but does not show interference until convoluted at the screen. If you move the screen further out or closer in, that’s where the photons will convolute and form the interference pattern. $\endgroup$
    – Bill Alsept
    Nov 6, 2017 at 23:30

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The fringes which you have described are non-localised, they occur everywhere where there is which has passed through the slit(s).

When you use your eye to observe the fringes, you are observing them from the slit(s) being focussed on the retina of the eye, just like the telescope of the spectrometer focussing the light in the focal plane of the telescope objective lens.
The eye then acts as a magnifying glass to make the fringes appear larger.

Without the telescope your eye probably is focussed on the slit(s) as it is very difficult to focus on "thin air".
However, you could help the eye do this by placing a translucent sheet between the slit and the eye to observe the fringes formed in the vicinity of the paper.

If you used a laser as the source of light it is easy to see the fringes wherever you put a screen.

This photograph of the waves from two vibrating sources in a ripple tank show the the non-localised formation of interference fringes.

enter image description here

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