Electron beam shot to a "thin copper foil". At some distance behind the foil there is a fluorescent screen.
In order to observe a ring diffraction pattern on the screen shown above, which of the following conditions must be met?
A) the electron beam must be polarized
B) the electron beam must be approximately monoenergetic
C) the copper foil must be a single crystal specimen
D) the copper foil must be of uniform thickness
E) the electron beam must strike the foil at normal incidence
The correct answer was given as (B). Can anybody gives explanation about this? Thanks so much in advance.