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I was learning about the double slit experiment and simple explanation is that there is wave interference. Although I do not quite understand the wave bit. We know that light is a wave. This Sine type wave

But in the image of the wave interference we see something like two ripples (in water) that interfere. Wave interference

I couldn't understand that how could a sine wave, pass through a slit and then suddenly become ripples.

P.S. If we imagine a sine wave passing through a slit, we (or probably I) should expect a sine wave, because the front side of the sine wave is one-dimensional and that passing through a slit just a wee wide, shouldn't change anything.

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  • $\begingroup$ Are you asking how the light is able to spread out as it exits the slit? $\endgroup$
    – S. McGrew
    Commented Aug 31, 2020 at 17:50
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    $\begingroup$ The lines you see represent the maxima of the sine probability waves. $\endgroup$
    – Cosmas Zachos
    Commented Aug 31, 2020 at 17:59
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    $\begingroup$ @Shubhkarman Sandhu those curves escaping the slit represent the wavefront not the sine wave itself. $\endgroup$
    – Ankit
    Commented Aug 31, 2020 at 18:21
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    $\begingroup$ The cross-section of a series of ripples is a sine wave. This is true whether the cross-section is longitudinal or radial and the ripples are in light, water or anything else. $\endgroup$
    – Guy Inchbald
    Commented Aug 31, 2020 at 18:54
  • $\begingroup$ The probability of finding the photon at a place when measuring is related to the (operator-)weighted inner product of the photon's wave function, which is a complex number at each position, with dynamics as the Schrödinger equation tells. I don't know if the first picture is supposed to show the wave function or the electric field. $\endgroup$
    – Emil
    Commented Sep 1, 2020 at 5:54

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As the wave passes through both slits the light diffracts into two coherent waves. Try to imagine the sine wave graphic (top picture) but from above. It's the same principle as a ripple; it's just crests and troughs. The lower graphic can be misleading as when you'd do this experiment with a laser there isn't nearly that much spread of the beam before it encounters the slits.

https://en.wikipedia.org/wiki/Diffraction

enter image description here

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