If EM waves are not physical, positional waves (on a X,Y,Z axis), why does interference pattern appear positional? I have read that EM waves propagate in straight lines:
https://van.physics.illinois.edu/qa/listing.php?id=17699
Wherein the only the electric (E) and magnetic (B) fields to change (or oscillate) at corresponding locations, not the location of the photos or the photon packets themselves.  The photons/packets propagate in a straight line.
Is this correct?
If so, why does the interference of two EM waves in the double-slit experiment seem to show positional, ending location of the interference patterns? Is each "dot" the measurement of where the photon "landed"?  or do the photons land everywhere, and only the ones with high energies (combinations of wave peaks) are displayed as brighter "dots"?  
If you watch videos like these, they show that the interference of the two Em waves causes the actual physical location of the absorbed photos to change according to a physical wave, like you might see with water waves.  
So now I am confused by what the interference pattern "dots" actually show? Localization phase interference of physically propagating waves and the resulting locations of the impacting particles?  Or merely the EM interference of the EM waves represented as darker or brighter dots (wherein the particles are hitting everywhere)?
i thought 1 particle hitting the detector = 1 equally "bright" dot -- indicating the exact location of that absorbed particle?  Or does every dot on the detector get hit and only the brightness varies by energies?
Or something else?  Thanks
 A: Classically EM radiation is just a wave (not a particle) and the double slit experiment is only the result of the well known interference proprieties of waves.In quantum mechanics we've got the wave-particle duality, and so the light could be seen as composed by particles, photons. So the first thing to point out is that mixing the two approaches (talking of photons in classical electrodynamics) could be done only with caution. Back to your question: in the real world light propagates in spatially finite packets and saying that EM waves are extended in the whole space is just an approximation (which has several problems, like infinite energy). So when you see a bright spot you are actually seeing a photon, dark spot: no photons. The interference patterns emerges due to quantum mechanics wave-particle duality. To make you clearer this point: the double slit experiment has been performed also with electrons, the dots you see in the image are actual electrons, but there is also an interference pattern.   
