Young slits, are interferences possible without diffraction? Is it possible to have a Young experiment with N slits producing interferences but no diffraction?
In my course, we have two formula's for the intensity at a certain point after the slits: one for interferences and another for diffraction (which actually include the interference formula as one of his factors), but .. isn't it necessarily the second one happening ? Is it possible to have the first one only? If yes, how is it possible? (It seems to me that the experiments are the same)
 A: There is always diffraction .... photons (a localized traveling wave in the EM field) interact with the EM field of the aperture or slit(s).
It is possible to have just diffraction and NO interference .... when water goes thru a single slit it merely diffracts .... interference is only seen if we allow the waves to bounce off the wave tank edge.
Interference is a historical and somewhat inaccurate word ... photons never cancel each other as it would be a violation of conservation of energy.  Photons determine there preferred paths independently ... the Feynman path integral does a good job of calculating the slit probability distribution.
A: To get interference, the the waves through the slits have to overlap at the detector. For a plane wave in, that's not geometrically possible without diffraction.
But for a converging wave, the waves through the slits will meet at the focus. You'll get interference there. Of course, you'll always get diffraction, regardless, but you'll get more intensity at the detector in this case.
You may also put a lens downstream of the slits to accomplish the convergence. This is a common strategy in grating spectroscopes.
A: Light is often traced through optical systems using rays. This treats light as if it traveled in a straight line from one lens surface or other optical element to the next, and then instantly changes direction.
This is a good approximation, but light is a wave. It doesn't travel in straight lines. For example, it bends around obstacles such as slits.
It sometimes bends in vacuum far from matter. It isn't intuitive, but it is real. This is common in laser beam. The bending is typically slight, but it is there. See Gaussian Beam. The shape of the beam is calculate by solving Maxwell's Equations, using the curved laser mirrors as boundary conditions.
Diffraction can be loosely defined as the difference between the approximate straight line ray tracing behavior and the real wave behavior.
You can arrange for light to travel without diffraction. You just need to arrange special circumstances where it truly dos travel in straight lines.
Light from a distant start does this to as good an approximation as we can measure. The star is so far away that we cannot distinguish it from a point source. Light from a point source travels straight outward in an expanding sphere. For a star, the sphere is so large that you can't measure the difference between the portion we see and a true plane. A plane wave travels light a bundle of parallel rays.
If you pass starlight through a pinhole, it will diffraction. So don't.

You can get interference by arranging two beams of light to intersect.
You might think light from two different stars would do the trick. But there is a problem. Starlight was created by hot atoms. You get a diffraction pattern. But a nanosecond or so later, light from different atoms arrives and the diffraction pattern changes. It blurs so fast you can't see it.
Lasers help here. They have coherent light where the diffraction pattern stays stable long enough to see. You have to split the beam of a single laser in two and then combine them.
One way is to pass light from one beam through two slits. But that creates diffraction.
We could do it like stars do. Let the beam spread way out until it is going straight. Then use a giant beam splitter and giant mirrors to split the beam in two and make the two intersect.
If you are willing to be satisfied with only a little diffraction, you can use ordinary sized optics. If you pass a beam through a small pinhole you get a lot of diffraction. You can think of a lens as filling a very large pinhole. You get a little diffraction.
