Chemistry Reference
In-Depth Information
3
Addition of the condenser converts the incident parallel beam into a
cone beam and creates an interference figure that can be observed by
either removing the ocular or by adding the Bertrand lens. There is no
particular name for such observation in English; in French it is called
“
observations in convergent light”
(LC).
Although opaque minerals are not determined with the transmit light
microscope we may still have some indication in considering the form of
opaque minerals (cubic habit for magnetite and pyrite, elongated habits for
ilmenite, for instance) and also by removing the lamp from its slot and illu-
minating the thin section from above: One recognizes the yellow brass color
of pyrite, bright yellow to orange of chalcopyrite, the grays of ilmenite and
magnetite. This does not replace a serious determination with the reflected
light microscope.
2.3 CRYSTALLINE PLATE WITH PARALLEL
FACES IN CROSSED POLARIZED LIGHT
Observations under the petrographic microscope are interpreted by the
interaction between a plane polarized light wave and a plate with parallel
faces of a given mineral.
Let us consider a plate with parallel faces characterized by the perpen-
dicular directions in which the light propagates without distortion at veloci-
ties c/n
γ
and c/n
α
. These two directions we will be used as coordinate axes
Ox and Oy.
The incident vibration, after the (lower) polarizer, is plane polarized:
OP
=
a sin2
π
t/T
where a is the amplitude of the vibration and T its period.
This vibration makes an angle
with the Ox axis.
If we decompose this vibration on the two axes Ox and Oy
α
Ox
=
a cos
α
sin2
π
t/T
Oy
=
a sin
α
sin2
π
t/T
This vibration passes through the plate of e thickness with a velocity
c/n
γ
along Ox and velocity c/n
α
along Oy. The duration of the crossing
will be (e n
/c) along Oy.
At the exit of the plate, the equation of the vibration will be:
γ
/c) along Ox and (e n
α
Ox
=
a cos
α
sin2
π
(t
−
e n
γ
/c)/T
Oy
=
a sin
α
sin2
π
(t
−
e n
α
/c)/T
