Dear Crystallographers,
Here are a few paradoxes about diffraction I would like to get some
answers about:
1. In every description of Braggs' law I've seen, the in-coming waves
have to be in phase. Why is that? Given that the sources used for
diffraction studies are mostly non-coherent.
2. Trying to derive the diffraction condition for a pair of non-coherent
waves with a phase difference of 'y' where 0 < y < 2pi, I obtain the
following diffraction condition
y * (lambda/2pi) = 2d sin (theta)
i.e. the phase difference y = 4pi * sin(theta) * d / lambda
This seems to imply that diffraction will occur if the incident waves
are not in phase but the phase difference still satisfies the above
condition. One may be able to envision a case where for a given distance
d, the diffracting condition will be met for various angles depending on
the phase shift of the waves diffracting. Does this make sense? Has
anyone looked at the significance of this relationship before? Any
pointers will be welcome.
3. What happens to the photon energy when waves destructively interfere
as mentioned in the text books. Doesn't 'destructive interference'
appear to violate the first and second laws of thermodynamics? Besides,
since the sources are non-coherent, how come the photon 'waves' don't
annihilate each other before reaching the sample? If they were coherent,
would we just end up with a single wave any how? With what will it
interfere to cause diffraction?
I'm sure some of these may have some really obvious answers I may be
missing.
Thanks,
Michel
