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Last week I performed Young's double slit experiment using a laser. As expected, I obtained an interference pattern as predicted by Fraunhofer theory (enveloped by the 1 slit diffraction curve). Then, I added two polarizers with perpendicular axes in front of each slit. Since the light coming from the slits had now different polarizations, I no longer noticed the interference pattern (only the diffraction from each separate slit).

It is generally known that in order to perform Young's double slit experiment using natural light, we should first make the natural light "more" coherent by placing a single slit before the double slit. However, from my understanding, natural light is unpolarized. So now my question is, why does natural light produce an interference pattern if the incoming light in unpolarized? Shouldn't the light from the 2 slits not interfere because the polarizations are different just like in the laser experiment? Does the first slit polarize the light? Or is it that the light exiting the 2 slits is synchronously unpolarized (the electric field vectors are always on the same line) hence creating a pattern?

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Both polarizations destructively and constructively interfere at the same points, meaning their sum also shows the diffraction pattern. This is why unpolarized light can be used in Young's experiment with no problem.

However, by letting a different polarization to pass in each slit, you force a situation where each polarization doesn't have two sources to interfere. (meaning there is a sum of two diffraction patterns from a single slit that can't interfere)

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    $\begingroup$ maybe you should be clearer in your first paragraph, which is the answer to "Young's double slit experiment using natural light, we should first make the natural light "more" coherent by placing a single slit before the double slit.".. The second paragraph refers to the OP's experiment. $\endgroup$
    – anna v
    Commented Nov 16, 2021 at 19:31
  • $\begingroup$ @annav You're right! I'll try to fix this $\endgroup$
    – Ofek Gillon
    Commented Nov 16, 2021 at 19:49
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Unpolarized light has a polarization vector which varies randomly with time, so at any given time, the polarization at the left and right slots will be the same (and thus they will interfere). However, natural light also has low spatial coherence (i.e. the phase varies across the wavefront over a relatively short distance). So this incoherence can manifest as a polarization discrepancy between slits (not likely a 90$^\circ$ discrepancy if the slits are close; generally something less).

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