Electron diffraction from crystals

[blueyellow]

why is it that in electron diffraction from cystals, theta i and theta r are not necessarily equal?

 

[Gokul43201]

What are theta i and theta r? Define carefully.

 

[blueyellow]

theta i is the angle of incidence
theta r is the angle of reflection
a diagram of it can be seen at:
http://instructor.physics.lsa.umich.edu/adv-labs/Electron_Diffraction/electron_diffraction2.pdf

 

[Bill_K]

blueyellow,

The angle of diffraction is determined by the Bragg condition, namely that the difference in path length from one atom to the next is an integral number of wavelengths. The writeup you refer to does assume that θ_i = θ_r, but this is only a special case. If you draw the same diagram and let θ_i and θ_r be different, you'll see that the Bragg condition becomes

d(sin θ_i + sin θ_r) = nλ

(Most references use instead the angle that the electron's path makes with the normal, in which case replace sin by cos.)