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Sorry for asking this in here, but since the double slit experiment is mentioned, I have a question. Is the wave formed by light a 2d wave, like waves on the s

Sorry for asking this in here, but since the double slit experiment is mentioned, I have a question. Is the wave formed by light a 2d wave, like waves on the surface of the water? And if so, then if we turn the slits sideways, why does the interference pattern also turns sideways? it shouldn't turn sideways if light is a 2d wave. My apologies for the offtopic question, but I couldn't find any answer about this anywhere.

pjc50 8y ago
The "direction" of the wave affects polarization, which is critical for LCDs working. But that's not relevant to the double slit experiment, because the slits and spacing are larger than the waves. The double slit experiment relies on diffraction, the tendency of the waves to bend round corners.
TheOtherHobbes 8y ago
Light acts like a 2D wave when it's linearly polarised. But it can also have circular polarisation - which rotates - or some mix of polarisations. If you put a vertical polariser in front of one slit and a horizontal polariser in front of the other slit, the usual pattern of fringes disappears. So polarisation does matter for the experiment. But I'd strongly suggest not thinking of light in wave and/or particle terms. In Quantum Field Theory everything is made of fields, which are more like carriers of probability which strongly hint at some weird non-local properties, than a nice smooth lake surface waiting for some ripples. If you try to apply physical intuition to QM it soon stops making any sense at all. So IMO it's best not to start there.
- axilmar 8y ago
  If we can't make the analogy between the surface of water and light, then how come the double slit experiment shows us that the interference pattern is due to the wavy nature of light?
  - spiralx 8y ago
    Because a wave is a more general phenomenon than what happens on the surface of water, and waves of all kind result in interference patterns. Also, waves in the ocean certainly aren't two dimensional either, the surface "wave" just a visible manifestation of a larger system. But the polarisation of light demonstrates more quantum effects. Put two polarising filters in front of a light at 90 degrees to each other and no light gets through them both; yet add a third filter in between at 45 degrees angle to each, and now half of the light gets through all three filters! Adding extra filters can make them let more light through, not less... http://alienryderflex.com/polarizer/ http://alienryderflex.com/polarizer/
abdullahkhalids 8y ago
Good question. The pattern will indeed turn sideways. 1. There is nothing deep about the shape of the slit. You can do the "double-slit" experiment with "double-pinhole" or "double-weirdshapes" and you will get the same behavior, just that the math will be different. 2. In fact, there is a simpler experiment you can do which gives the same qualitative results, but in which the shape of the wave is much less relevant. You can split a single photon into two fiber cables using a beamsplitter, then interfere the light in those cables using another beamsplitter. The good thing about this is that all the operations in this setup are linear, because light is not lost anywhere. In a double-slit experiment, light that falls outside the slits gets blocked and so the operations are non-linear and you have to worry about the shape of the slits and wave a lot more. 3. I am not quite sure which two dimensions you mean, when you say 2d wave. There are two axes in the plane of the slits (longitudinal) and another axis perpendicular to this plane (transverse). The wave of light is in fact 3D, but lets simplify. If its 2D in the longitudinal axes, then obviously the shape/orientation of the slits matter. Imagine putting a rainbow colored piece of paper against the slits. Depending on the orientation of the slits, different colors will make contact with the slits. If its 2D in transverse+one longitudinal axis then it still matters. Imagine taking a stack of identical rainbow colored paper and put it edge on against the slits. If you rotate the slits, obviously the colors making contact with the slits will change. Hence, the interference pattern changes again.
- axilmar 8y ago
  So, the wave of light is actually 3d? then if it is 3d, why don't we see the vertical bars in the interference pattern like this ? - - - -.
  - abdullahkhalids 8y ago
    1. Yes, light is always 3D but the description in popular science is always simplified. Also experiments usually measure along a line across the screen and don't build up a 2D pattern on the screen that you usually see in popular science. For instance Fig 4. here [1] . 2. Because the light does not diffract up and down too much, only horizontally. So, for instance, the light emerging from the middle of the left slit stays at the middle level and does not interfere with the light emerging from the bottom of the right slit. Imagine looking somewhere on the screen at middle height where the path lengths are so that its a dark point. The light here is coming from the middle of both slits. Now move down a bit. The light here is coming from the bottom of the two slits here, and the path lengths are still the same. So you still get a dark point. I hope this makes it clear. [1] http://elib.bsu.by/bitstream/123456789/154548/1/1991-066%20PRL%20Carnal%20%26%20Mlynek%20-%20Young%E2%80%99s%20double-slit%20experiment%20with%20atoms%20-%20a%20simple%20atom%20interferometer.pdf http://elib.bsu.by/bitstream/123456789/154548/1/1991-066%20P...

pjc50 8y ago

The "direction" of the wave affects polarization, which is critical for LCDs working. But that's not relevant to the double slit experiment, because the slits and spacing are larger than the waves. The double slit experiment relies on diffraction, the tendency of the waves to bend round corners.

TheOtherHobbes 8y ago

Light acts like a 2D wave when it's linearly polarised. But it can also have circular polarisation - which rotates - or some mix of polarisations. If you put a vertical polariser in front of one slit and a horizontal polariser in front of the other slit, the usual pattern of fringes disappears. So polarisation does matter for the experiment. But I'd strongly suggest not thinking of light in wave and/or particle terms. In Quantum Field Theory everything is made of fields, which are more like carriers of probability which strongly hint at some weird non-local properties, than a nice smooth lake surface waiting for some ripples. If you try to apply physical intuition to QM it soon stops making any sense at all. So IMO it's best not to start there.

axilmar 8y ago
If we can't make the analogy between the surface of water and light, then how come the double slit experiment shows us that the interference pattern is due to the wavy nature of light?
- spiralx 8y ago
  Because a wave is a more general phenomenon than what happens on the surface of water, and waves of all kind result in interference patterns. Also, waves in the ocean certainly aren't two dimensional either, the surface "wave" just a visible manifestation of a larger system. But the polarisation of light demonstrates more quantum effects. Put two polarising filters in front of a light at 90 degrees to each other and no light gets through them both; yet add a third filter in between at 45 degrees angle to each, and now half of the light gets through all three filters! Adding extra filters can make them let more light through, not less... http://alienryderflex.com/polarizer/ http://alienryderflex.com/polarizer/

abdullahkhalids 8y ago

Good question. The pattern will indeed turn sideways. 1. There is nothing deep about the shape of the slit. You can do the "double-slit" experiment with "double-pinhole" or "double-weirdshapes" and you will get the same behavior, just that the math will be different. 2. In fact, there is a simpler experiment you can do which gives the same qualitative results, but in which the shape of the wave is much less relevant. You can split a single photon into two fiber cables using a beamsplitter, then interfere the light in those cables using another beamsplitter. The good thing about this is that all the operations in this setup are linear, because light is not lost anywhere. In a double-slit experiment, light that falls outside the slits gets blocked and so the operations are non-linear and you have to worry about the shape of the slits and wave a lot more. 3. I am not quite sure which two dimensions you mean, when you say 2d wave. There are two axes in the plane of the slits (longitudinal) and another axis perpendicular to this plane (transverse). The wave of light is in fact 3D, but lets simplify. If its 2D in the longitudinal axes, then obviously the shape/orientation of the slits matter. Imagine putting a rainbow colored piece of paper against the slits. Depending on the orientation of the slits, different colors will make contact with the slits. If its 2D in transverse+one longitudinal axis then it still matters. Imagine taking a stack of identical rainbow colored paper and put it edge on against the slits. If you rotate the slits, obviously the colors making contact with the slits will change. Hence, the interference pattern changes again.

axilmar 8y ago
So, the wave of light is actually 3d? then if it is 3d, why don't we see the vertical bars in the interference pattern like this ? - - - -.
- abdullahkhalids 8y ago
  1. Yes, light is always 3D but the description in popular science is always simplified. Also experiments usually measure along a line across the screen and don't build up a 2D pattern on the screen that you usually see in popular science. For instance Fig 4. here [1] . 2. Because the light does not diffract up and down too much, only horizontally. So, for instance, the light emerging from the middle of the left slit stays at the middle level and does not interfere with the light emerging from the bottom of the right slit. Imagine looking somewhere on the screen at middle height where the path lengths are so that its a dark point. The light here is coming from the middle of both slits. Now move down a bit. The light here is coming from the bottom of the two slits here, and the path lengths are still the same. So you still get a dark point. I hope this makes it clear. [1] http://elib.bsu.by/bitstream/123456789/154548/1/1991-066%20PRL%20Carnal%20%26%20Mlynek%20-%20Young%E2%80%99s%20double-slit%20experiment%20with%20atoms%20-%20a%20simple%20atom%20interferometer.pdf http://elib.bsu.by/bitstream/123456789/154548/1/1991-066%20P...