Should light travelling post-double slit between an elongated single slit formed from two razor blades still interfere?

Question

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.

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.

Answer 1

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.

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.

Answer 2

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.

Answer 3

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.