# Electron Gun diffraction and undeflected spot diameter

• I
Hi I am doing an electron gun diffraction right now and I wonder why the angle below is 2 phi. What I think about it that it's actually the results of two waves one is shown below and the other is a wave
with the same accident angle but negative "those two rays will be symmetric around the lattice plane" and all other rays will cancel each other. Thus will image in the screen is the result of the two rays and the angle between them will be 2phi.
Even-though this makes sense to me, but I want to know is it true. and is there a proof that all the other rays"other than the twos" will cancel each other #### Attachments

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sophiecentaur
Gold Member
If the ϑ in the top diagram is the θ in the bottom then the change of direction in the bottom is θ+θ. This is general and is true for rotation of an optical mirror. The image moves twice as much as the mirror rotation.
For Bragg scattering, the only directions where you get constructive interference is determined by the spacing d. So for other values of θ you are not at a max.

• rashida564
If the ϑ in the top diagram is the θ in the bottom then the change of direction in the bottom is θ+θ. This is general and is true for rotation of an optical mirror. The image moves twice as much as the mirror rotation.
For Bragg scattering, the only directions where you get constructive interference is determined by the spacing d. So for other values of θ you are not at a max.
Is there a mathmatecal proof behind this

Like why is it possible to think about it as a rotates mirror

sophiecentaur