Is it possible for light to be diffracted more than ninety degrees?

When light diffracts through a narrow slit or pinhole, it travels in all forward directions. The width of the slit determines how much the light will cancel itself.

Light travels from each point in the slit to everywhere. Light from different points in the slit must travel different distances. So it will be in different phases. At some spots all the light cancels with light in different phase. At other spots only some of it cancels.

Obviously you can't detect light that is bent more than 90 degrees, because the flat wall the slit is in would absorb it if there was any such thing.

What if the wall wasn't flat? What if we had, say, two razor blades that make the slit between them, and they are turned at 90 degrees to each other.

Is there theoretical reason to say that light cannot diffract backward?

Has the experiment been done?

enter image description here

  • $\begingroup$ This is just to say I see no reason why more than 90 degrees wouldn't be possible and I expect it is possible. $\endgroup$ – Andrew Steane Dec 6 '18 at 13:51

Yes it is possible. Your explanation of diffraction is an historical one, modern explanations of diffraction don't have light cancelling out. Instead photons prefer to travel a path of length n times their wavelength where n is an integer. Dark areas are where no photons have landed and bright areas are where most of the photons have landed. (Lots if explanations on this site, you can also google what are known as single photon diffraction experiments.) The intensity pattern is a result of QM or probabilistic interactions of the photon EM field with the EM fields of the slit material. So yes its possible but not very probabale.

  • $\begingroup$ Thank you! As I understand it, the wave and photon explanations give the same results in all circumstances. When the light intensity is low, we face the fact that all light detectors are themselves quantized and for low light intensities have a probability per unit time of detecting the light wave. So we get the same intensity pattern that QM predicts. Since the result is the same, I prefer to use the description that is easier to follow. Thank you for saying it's possible. $\endgroup$ – J Thomas Dec 6 '18 at 14:21
  • $\begingroup$ The simplified formula to predict where on a flat wall to predict intensity minima is $y = \frac{m \lambda D}{a}$ where $\lambda D a$ are constants. That says an infinite number of minima will be spread out on the forward wall with none of them beyond 90 degrees. But it's an approximate formula and not something I can depend on to extrapolate outside its range. $\endgroup$ – J Thomas Dec 6 '18 at 14:54
  • $\begingroup$ Yes the "classical explanation" is not bad, but it leaves many people thinking that 2 photons cancel or annihilate each other which is not possible. If 1000 photons go thru the slit then 1000 photons are observed on the screen. $\endgroup$ – PhysicsDave Dec 6 '18 at 15:07
  • $\begingroup$ "interactions of the photon EM field with the EM fields of the slit material" - This is incorrect. Photons don't interact with the EM field. $\endgroup$ – safesphere Dec 6 '18 at 15:33
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    $\begingroup$ Photons travel in a vaccuum therefore the EM field was theorized to carry the energy, so you can say a photon is a ripple in the EM field just like a water wave is a ripple in water. So we can say the EM field is there without a photon but it's just semantics. But we can observe a calm pond and and one with a ripple separately. But don't let that confuse the point about cancelation of photons, they don't cancel each other, especially true for diffraction. $\endgroup$ – PhysicsDave Dec 6 '18 at 19:30

Good question. Here's a photo I just took. It's a razor blade pushed into some wood. You can see the light wraps around the blade well beyond 90 degrees.enter image description here

  • $\begingroup$ That does look like more than 90 degrees! How did you make the laser light visible? Did you blow smoke at it? $\endgroup$ – J Thomas Dec 6 '18 at 19:25
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    $\begingroup$ No smoke just a laser on a razor blade. $\endgroup$ – Lambda Dec 6 '18 at 19:35
  • $\begingroup$ I'm real unclear what you're seeing. I see a bright green line with a faint green fog around it. The green line looks like it's where green laser light hits the wooden board, and it's pretty much exactly in front of the laser. There's a green dot on the razor that the camera picks up. That dot should have come from the laser, and to see it at all it would have to be bent around the razor blade at an extreme angle backward and also sideways a lot. Is that green dot the light wrapped around the blade? If it wasn't thoroughly wrapped around we wouldn't see it at all? $\endgroup$ – J Thomas Dec 6 '18 at 22:37
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    $\begingroup$ Yes, it’s clearly visible without a camera. If you darken the room you will see a clear line of reflection that goes well beyond 90. It most likely goes 360 but as it nears the source beam it gets lost in the glare. $\endgroup$ – Lambda Dec 6 '18 at 22:46
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    $\begingroup$ Yes, that’s the way I see it too. $\endgroup$ – Lambda Dec 8 '18 at 0:28

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