208
9.
X-RAYS
graphic plate P. Upon suitable exposure and development of the
plate a series of spots
Will be found around the central region where
the direct beam reached the plate.
the case of x—rays passing
through a rock salt crystal, normal to its cleavage plane or cube
face, the photograph Will appear as in gure ()
at
the right.
The Laue spots
lie on ellipses, all of which pass through the center
p
C
\
'
l
r
——
—
—
—
A
B
/
(
©
D
’
'
“
“'
b
l «———
FIG. 9—13.
Laue apparatus and spots.
of the pattern and whose major and minor axes are the axes /) and 5
of the crystal; the plane of the photographie plate is parallel to
the bc plane of the crystal.
The planes in a crystal are customarily identied by means of
three numbers, called the Miller indices, /z, /e, and /.
let three
axes be drawn in the crystal, along rows of atoms, as in gure 9—14
2
Z
Z
y
‘ êîl
°
V
y
x
x
x
(|…)
planes
(om) planes
(236) planes
FIG. 9—14.
Miller indices (I:/ and let a, & and c be the grating constants or distances between
the atoms along the x,
and z axes, 1“6Spectively.
'l‘he Miller
indices are the smallest integers proportional to the reciprocals
of
the mtercepts of the planes on the axes.
Thus, if the intercepts
of a plane With the axes are 3a, 2& and lc, as in the right hand
gure of 9—14, their reeiprocals are
1/2& and 1/15 or 2/6a,