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I am complete dilettante in physics, when i was first introduced to quantum mechanics by a You tuber named Arvin ash, he explained about the double slit experiment with quantum particles, there in his video when the which way path of the particles is detected formed a double slit pattern it was just a two strips in the video he made, but when i gone through a paper regarding Young's double slit experiment with single photons the double slit pattern was just a blurred image with no interference, but why is that instead of a two strip pattern the double slit pattern is just a blurred image with no interference?

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  • $\begingroup$ Fig 5 has nothing but figures 6 7 8 9 10 11 12 all show interference pattern! $\endgroup$ Dec 17, 2022 at 23:05
  • $\begingroup$ Re, "a two strip pattern..." Are you asking why you don't see a sharp-edged projection of the two slits onto the screen? The answer to that is diffraction. A wave (any kind of wave, light wave, water wave, etc.) passing an obstruction will be diffracted by the obstruction (it wraps around the edge.) When passing through an opening that is less than about 4x the wavelength in width, the amount of light that is able to go "straight through" becomes less than the amount that gets diffracted by the two edges. $\endgroup$ Dec 17, 2022 at 23:08

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Without interference, the energy intensity at a point on the screen is proportional to $A^2+B^2$, where $A$ and $B$ are the amplitudes of the waves coming through each slit individually. With interference, it's proportional to $A^2+B^2+2AB\cos θ$, where $θ$ is the phase angle between the waves from the two slits.

If $A\approx 0$ or $B\approx 0$ at a point, then $AB\approx 0$, and the intensity will be about the same whether there is interference or not. So while it is possible to set up your light source and slits such that the light from one slit scarcely overlaps with the light from the other on the screen, you are not really doing the double-slit experiment if you do that.

Any introduction to the double-slit experiment that says "with a detector at the slits, you get two stripes, and without the detector you get an interference pattern" is incorrect.

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