# Bragg planes seem imaginary

1. Aug 19, 2011

### ninevolt

I am having trouble understanding what exactly Bragg planes are physically.
I understand how they behave, in that they act like mirrors and reflect matter waves, but what exactly is the wave bouncing off of?

for instance

I can guarantee any physics textbook always has a picture like this, with the light bouncing off of what it seems to be nothing physical, not an atom, just the Bragg plane. I am really having trouble swallowing what exactly causes the Bragg plane. Or is it something that we just accept like electrons not emitting radiation in atoms even though they seem to be accelerating?

2. Aug 19, 2011

### JeffKoch

Well, what do you think the little spots in the diagram are? Reflection, even normal reflection off a mirror, is a scattering problem arising from groups of atoms responding to incident EM radiation - so the "bouncing off" analogy is flawed. A Bragg reflection plane is a plane in a crystal containing some significant fraction of the atoms that make up the crystal, and generally there are many such planes in a crystal depending on the symmetry of the atoms - the diagram you posted shows several, there are many more including many that are tilted into and out of the plane of the diagram. There's nothing unphysical or imaginary about them.

3. Aug 19, 2011

### ninevolt

So are you saying that in real life the wave will reflect off of the atoms in the plane? And that macroscopically the atoms are close enough together to constitute the Bragg plane?

If this is the case, I still do not understand why the waves bounce off of the atoms as if they are in a plane. Atoms are round, and to my knowledge we would get a different reflection pattern that that of a simple plane.

4. Aug 19, 2011

### Bill_K

Bragg reflection happens when the wavelength of the incident light is much larger than an atom. For high frequencies the light will impact a single atom, but for lower frequencies it affects many neighboring atoms. What does an individual atom see? It sees a uniform, slowly varying E field. When exposed to a uniform E field, an atom will become polarized, i.e. acquire an electric dipole moment. As the E field slowly varies, the dipole moment slowly varies, reradiating the light in all directions. Along the direction of a Bragg plane, the reradiated wavelets are in sync with each other and add constructively.

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