Page 1 of 1

ETS extra, Q#13

Posted: Mon Oct 19, 2009 6:33 am
by hk
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.

Re: ETS extra, Q#13

Posted: Mon Oct 19, 2009 11:52 pm
by Prologue
I can't give a reason for the others being wrong but I can give a reason for that one being right. If the beam is not approximately monoenergetic then it means the electrons have different energies. If they have different energies then they have different wavelengths. If they have different wavelengths then the diffraction angles with be spread instead of localized and a pattern won't be recognized.

Re: ETS extra, Q#13

Posted: Tue Oct 20, 2009 11:22 pm
by hk
Thanks for your comments, that makes sense!

Choice (D) is rather ambiguous I think. I imagine if the thickness is greatly non-uniform, then strange patterns would appear.
Well I guess it is not necessary to be uniform as long as it's still a "thin foil" then.
The other choices may be quite easily eliminated.