3B Scientific Teltron Electron Diffraction Tube S - Manual (Page 7)

3
Two diffraction rings appear on the fluores-
cent screen centred on the undeflected beam
in the middle. The two rings correspond to
Bragg reflections from atoms in the layers of
the graphite crystal lattice.
Changing the anode voltage causes the rings
to change in diameter. Reducing the voltage
makes the rings wider. This supports de
Broglie's postulate that the wavelength in-
creases as momentum is reduced.
a) Bragg equation:
ϑ ⋅ ⋅ = λ sin d 2
λ = wavelength of the electrones
ϑ = glancing angle of the diffraction ring
d = lattice plane spacing in graphite
L = distance between sample and screen
D = diameter D of the diffraction ring
R = radius of the diffraction ring
L
D
tan
⋅
= ϑ
2
2
L
R
d ⋅ = λ
b) de-Broglie equation:
p
h
= λ
h = Planck’s constant
p = momentum of the electrones
m
p
U e
⋅
= ⋅
2
2
U e m
h
⋅ ⋅ ⋅
= λ
2
m = electron mas, e = electron charge
Fig. 1 Schematic representation to Debye-Scherrer diffraction