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

  • 2
    $\begingroup$ I've commented in other places that one should not try to comprehend the quantum and the classical picture at the same time until you have mastered each picture separately. The two ways of talking about physics involve naively similar concepts that are very different, so you only end up conflating them and confusing yourself. Classical waves are waves which means that they locally propagate in straight lines, but they are space filling entities which obey Huygens principle. Photons obey a different wave equation which has different meaning. $\endgroup$ Commented Jun 7, 2015 at 17:50

1 Answer 1


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,enter image description here the dots you see in the image are actual electrons, but there is also an interference pattern.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.