In the double slit experiment you can see the electromagnetic waves as curved lines that go through both slits and interfere with each other.

But, don't electromagnetic waves look like this.

So, three questions

1: What do the curved lines in the double slit experiment represent, is it the EM waves?, if so why do they are they like that?

2: Why can light be thought of as a water wave that propagates outwards in every direction? (like in the double slit experiment)

3: I heard If you were to put a detector on the two slits and see which slit the photons went through the photons wouldn’t display any interference pattern is this true? If so what causes it?

Thanks in advance for the replies.

  • $\begingroup$ The first image is bad because the wave fronts on the right are not coming from the centers of the slits. $\endgroup$
    – Jasper
    Commented Aug 18, 2018 at 20:54
  • $\begingroup$ The second image is a superposition. The fields don't oscillate, they rotate: physics.stackexchange.com/questions/360638/… $\endgroup$
    – safesphere
    Commented Aug 19, 2018 at 4:00

1 Answer 1


In your first link, the curved lines represent wave fronts: surfaces where the phase of the propagating wave is constant. The wave at a slit has the phase of the wave at that point entering the slit. Downstream, the phase depends on distance traveled from the slit, so the constant-phase surfaces are spherical. The diagram in the link shows a slice through the spherical wavefronts, hence the circular arcs.

In your second link, the image illustrates the relationship between the electric field and the magnetic field at a point in the field of a propagating EM wave. "Phase" would correspond to position along the axis, which is a time axis.

Almost any kind of wave is analogous to light waves. We use water waves as an analogy because everyone is familiar with water waves.

A good explanation of what causes disappearance of the interference pattern when anything is done to detect which slit a photon goes through requires a discussion of quantum mechanics. However, you could think of it this way: If you know which slit the photon went through, then it couldn't have gone through both slits. An interference pattern can only form if the photon does go through both slits.

  • $\begingroup$ One should be careful comparing light with water waves. A flash of ligh expands out leaving a complete darkness in the center. In other words, light does not reflect back from the empty space. The water waves behave in the opposite way. $\endgroup$
    – safesphere
    Commented Aug 19, 2018 at 3:56
  • $\begingroup$ A huge difference between light waves and water waves is that the speed of a water wave is greater if the wavelength is longer. That is not true of light waves moving in a vacuum. $\endgroup$
    – S. McGrew
    Commented Aug 19, 2018 at 20:50
  • $\begingroup$ Right, but what I mean is that the back wave (a reflection from the medium, including vacuum) cancels out only in an odd number of spatial dimensions. Your flashlight in a 2D or 4D universe would blind you by the light beam reflecting from the empty space back to you. Drop a rock in a lake (2D). The wave goes out in a circle, but the medium (surface) in the center is not left completely undisturbed as in 3D with light or sound. $\endgroup$
    – safesphere
    Commented Aug 19, 2018 at 22:05
  • $\begingroup$ I haven't read that water waves get reflected from undisturbed water and return to their source. Can you provide a link to a reference? $\endgroup$
    – S. McGrew
    Commented Aug 19, 2018 at 22:58
  • $\begingroup$ I have seen the "crown jewel effect" and similar things where a water drop falling into calm water can seemingly be re-formed and shot back upward. Behavior like that is due to nonlinearities in the water dynamics. As long as the disturbances are small, water waves behave in a "reasonable" way. $\endgroup$
    – S. McGrew
    Commented Aug 19, 2018 at 23:05

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