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I am continuing to experiment with Young's double slit experiment at home. As before I have used a small pen laser and shone the light through double slits formed from very fine wire and black masking tape. This gives the classical bright central dot and a series of bands stretching out on either side. However, I wanted to test whether it was possible to infer that the light was travelling as a photon rather than a wave by incorporating two razor blades positioned back to back very close together to make a 2 or 3 cm elongated single slit between the double slits and the screen. The gap between the blades is of the same order as the double slits. What I see is that the portion of the interference fringe which would have been there if the razor blades were absent is still present. If the light is acting as a wave as it travels from the double slits to the screen shouldn't this be prevented by forcing it to travel within the razor blades where it can't interact with the rest of the wave for 2 or 3 cm? It seems to be acting like a particle here. Can this be possible and if so is it breaking the Copenhagen interpretation?

Here is a diagram of my experiment. A spot of light reached the interference screen after having travelled between the two razor blades. The spot looked exactly like the one which would have been in the same place if the razor blades were not present. There was a small shadow on either side of it which was formed from light being blocked by the metal of the blades themselves.

Double Slit Experiment with single elongated slit from two razor blades

My query is essential this: Is the light acting as a photon as it passes between the razor blades? If so, does this mean that although the interference pattern fans out and so acts like a wave here, that in fact each path is made up of light acting as particles? And again, if this is so, where exactly does the interference take place? Maybe my next experiment will be to see how close to the double slits the razor blades can be placed without destroying the interference pattern? But I suspect that this needs doing with a single photon source and much finer equipment to really be definitive.

A. Kestner
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3 Answers3

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The razor blades are like a waveguide to the central interference spot. So it is not surprising that you will see the spot after all.

Here is a single photon at a time double slit experiment, which shows how the photons act.

single photon

The photons that were going straight at your two razor blades continue going straight.

Please note that photons do not interact with photons (very very weak intraction actually). It is the superposition of the photon wavefunctions, complex conjugate squared, that gives the probability distribution seen in the end accumulation.

anna v
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  • The spot formed from light travelling through the razor blades is not the central spot (although that occurs as well). As the razor blades are moved (and their angle aligned with the double slits) the spots of the interference pattern appear one after the other. Doesn't this imply that the observer knows the light is acting as a particle here and so what should appear (and because the slit is large enough for many photons) would be a single slit diffraction? – A. Kestner Nov 24 '16 at 11:41
  • No, after leaving the slit you just have some collimated light sources. either model with particles or optical rays it is fine. – anna v Nov 24 '16 at 12:40
  • I don't quite understand your answer. Isn't the light between the laser and the double slit collimated as well? Yet it manages to form an interference pattern. So as the light leaves the end part of the razor slit shouldn't this act as a single slit here and form an appropriate single slit pattern? – A. Kestner Nov 24 '16 at 13:07
  • The light leaving the laser is collimated and coherent, it is composed by zillions of photons in confluence. It hits the two slits and classically the two outgoing waves interfere ( like water waves from two holes), quantum mechanically single photons, as in the experiment above, leave a spot on a screen after the level of the slits, which builds up the interference. It is the two slits that create the interference pattern. A single slit has some diffraction at the edges but is centrally illuminated both classically and quantum mechanically, single photon at a time. – anna v Nov 24 '16 at 13:33
  • at the quantum mechanical level the photon wave function is the solution of the boundary value problem "photon + two slits" and this has sinusoidal variations that appear as the interference pattern when single photons build up . After the slit there are just beams of photons following the originating slit pattern. – anna v Nov 24 '16 at 13:35
  • v Right, if I understand your reply correctly then the light leaving the double slits is particles, photons, with their path determined by the interference on exit of the double slit. Their probability of hitting any particular part of the pattern on the screen is determined by the Schrödinger wave equation. I just wonder there must be a point with the two razors positioned very close to the double slit where the interference pattern must disappear. Wouldn't finding this point (with much better equipment than mine) tell us something about what is actually happening when photons interfere? – A. Kestner Nov 24 '16 at 13:46
  • but the simple double slit single photon tells us. Two photons do not interfere. Each photon wavefunction has a wavelike probability distribution governed by the square of the wavefunctions for the boundary conditions of double slits – anna v Nov 24 '16 at 15:32
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If I understand correctly, you are arranging the razor blades to allow only light to reach the screen only along a selected path.

If, peering through a peephole in the screen and back toward the slits, you can see only one slit, then interference will not occur at the location of the peephole. If you see reflections of both slits off the faces of the razor blades, though, there will still be interference.

The light and dark fringes on the screen (with no razor blades in the way) can be traced back continuously to a region relatively close to the slits. The bright and dark fringes, then, are like pages of a book filling the space between the slits and the screen. But light needs to come from both slits to interfere and form a fringe. If you could insert a thin black paper sheet just along one of the dark fringes all the way back from the screen to the slits, all of the interference would disappear because there would then be no place where light from both slits could reach.

An interesting fact emerges from this: in the location of a dark fringe, something is passing through even though no light can be detected there. That something is the vector potential.

S. McGrew
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You asked,

... Is the light acting as a photon as it passes between the razor blades? If so, does this mean that although the interference pattern fans out and so acts like a wave here, that in fact each path is made up of light acting as particles? And again, if this is so, where exactly does the interference take place?

The photon acts as both a wave and a particle whenever and wherever it is propagating. It can only be interpreted as a particle at the time and place where it is detected. In your experiment, the double slit produces a fan of collimated beams. If you block all beams but one, the one beam is just like the beam direct from the laser. Run that one remaining beam through another double slit and you'll get another fan of beams. You don't need any fancy equipment to demonstrate this.

S. McGrew
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