Chemistry Reference
In-Depth Information
1st order
2nd order
(very bright colors)
3rd order
(less bright colors)
4th order
(whashed out colors)
δ
= e (n
α
- n
γ
)
Figure 2.8
Michel-Lévy interference color chart.
indigo intermediate between red and blue (in French these colors are called
“teintes sensibles”
=
sensitive hues). In practice, one can easily distinguish:
1
colors of the beginning of the first order going from black to various
grays, more and more light up to white;
2
shades of the end of the first order: white, pale yellow, yellow, orange,
red;
3
bright colors of the 2nd and 3rd order: blue, green, yellow, orange and
red hues of the 3rd order are a little less vivid than the colors of the 2nd
order, but the difference is not very obvious;
4
washed out colors orders from higher orders: the iridescent beige, pink-
ish, greenish.
Warning! When a mineral has a strong color, the interference colors
are superimposed on the mineral's own colors and thus may not be very
recognizable!
Variations with
α
With the rotation of the microscope stage, four extinctions per turn are
observed, the extinction occurs for
α
=
k
π
/2. Extinction occurs when n
γ
and
n
of the section coincide with the planes of polarization of the polarizer
and analyzer (materialized by the cross hairs of the reticle). The maximum
intensity is observed at 45° of the directions of extinction.
α
Variations with e
The thin sections are cut to a standard thickness of 30 micrometers. If we
know the value of (n
) of the section of a given mineral, we can use
this data to measure the thickness e of the thin section. This property is used
by the technician to find out if the thin section is at the right thickness: at
30 micrometers: the interference color for quartz must be white (between
pale yellow and gray of the first order).
γ
−
n
α
