Double slit experiment and electromagnetic waves

Question

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.

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.