# Confusion about interference of light in Bragg's experiment

My current understanding of Bragg's experiment is this:

A collimated X ray beam was shone to the surface of a metal. If we model a metal as an aggregate of parallel planes, one X ray reflects off one layer, then another X ray reflects off a layer that is deeper. The point is that as a result of this, these two rays become out of phase. It is my understanding that when this phase shift is a multiple of 2 pi, the two light rays interfere constructively to produce bright patches on a detector.

Now here are my confusions. In high school, constructive interference of EMR was explained as the phenomenon which occurs when two crests of waves (in phase) superposed to produce an even larger crest, the amplitude of which is the sum of the amplitude of its constituent waves. How should I interpret the bright patches as a result of constructive interference in terms of photons?

Also, it was never explained to me how the two X rays can actually interfere with each other. As in, when they reflect off the crystal lattice, the two rays are still parallel to each other right? (It's just that now their phases are different). If the two rays are always parallel and never intersect at some point, how can they interference/ produce the bright patches on the detector?

English is not my strong point, so please comment if the question doesn't make sense as currently worded.

## 1 Answer

Your question actually is at the heart of the wave-particle duality, it just states that any particle (in your case a photon) can also be described as a wave, the latter feels interference in the same way you learned in high school.

About your second question, you can always argue that these waves are going to interfere at infinity, where you can place your detector. Infinity just means here a distance very large compared to $\lambda$, which in practical terms could be a few centimeters.

• So am I correct in saying that there's no particle explanation for the bright spots in diffraction patterns and to explain it we must resort to the wave model? – user166520 Sep 14 '17 at 14:30
• For your second point, why is infinity only taken to be large relative to the wavelength? Is it that the rays actually intersect at a few centimeters because the reflected light rays aren't perfectly parallel? – user166520 Sep 14 '17 at 14:32
• @J.Doe I think you can safely say that, but keep in mind that you cannot just forget about the photon is also a particle – caverac Sep 14 '17 at 14:33
• yes I agree with you that photons are "particles", but that's not really important here right? Since their particle nature doesn't offer any explanation about constructive interference – user166520 Sep 14 '17 at 14:35
• @J.Doe Your're right – caverac Sep 14 '17 at 14:40