,
3,_—6.
INDEX OF REFRACTlON OF METALS
55
in "the form V?” = 150, shows that the abscissae
of the
,
upper curve of gure 3—2 are inversely proportional to the wave—.
‘
length of
electrons.
The analogous action of_electrons and
x—rays is thus shown in this gure. Large electrometer readings,
-
the ordinates of gure 3—2, willbe obtained when the electron
waves, reected from the lattice planes of the crystal, are of the
appropriate length to cause constructive interference with each
other.
In other words, variation of the accelerating voltage, and
hence of the speed of the incident electrons, alters their wave—
length until the proper value is reached for selective reectionto
take place at the Bragg angle.
.
'
-
'
The wave-lengthof the electrons reected from a single crystal
.
_
in the Davisson and Germer experiment was Computed— from the
_
simple grating formula (3—7), using the known surface coristantn” -
_
for nicke‘land the Observed angles of incidence and difÎraction,
'_
-
B and 6’ .
Also, the wave—length of theincident electrons was
,
computed from the de Broglie equation. The two values were
found to be in excellent agreement. In_addition, the wave-length
‘
,
of the electrons which were reected from the lattice planes ina
'
_
thin surface layer was computed from the, simple Bragg fdrrhula
(3—8), using the
constantd f0r nickel and the obangle of diffraction
With the wave- '
,
length of the incident electrons,” as tOmputed from the de Broglie _
-
'
,
equation, the two values were found
in excellent agreement
,
,
for— the faster electrons
the
‘
—
trons.. The extent of
thQ degree ftÔ\ ;
Which the arroWs. fall»atthepeaksoftheCurve1ngure
>
“
)
*lsduet@aëmallchangemthewavelength0fthedecmnSastheY » Ï
,
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v
,
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,
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