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is
at least 0-02. A single plane facet is sufficient to give the whole of
the optical properties, except possibly the sign of the doable
refraction.
Optical groups
The
crystal systems could be divided into optical groups, e.g. (1) Cubic
and amorphous minerals—simply refractive or isotropic. These substances
have one refractive index and light travels with the same velocity in
all directions. These minerals are known as isotropic. (2) All
substances belonging to other crystallographic systems are anisotropic;
that is, the velocity of light varies with the direction in which it
passes through them. They may have one or two isotropic directions,
known as the direction of the optic axes. They are uniaxial, which
include the tetragonal and hexagonal systems which have one optic axis;
and biaxial—the remaining three systems, viz. the orthorhom-bic,
monoclinic, and triclinic crystals, because they have two optic axes.
For further details treatises on optical mineralogy could be consulted.
Pleochroism
The
colour of a substance is due to certain portions of white light being
absorbed in passing through it. This is also called selective
absorption. The portion not so absorbed causes the colour that is seen.
In isotropic substances light is absorbed equally in all directions.
But in anisotropic substances, the absorption of light varies with the
direction, as is also the case with the velocity of light. This
variation in absorption with direction causes the phenomenon known as
'pleochroism'.
Ruby
supplies a good example. When looked through parallel to the vertical
or optic axis, the ruby has a much deeper red colour than when viewed
in a direction at right angles to it. So rubies are better cut with the
table parallel to the basal pinacoid. Uniaxial crystals have two
principal indices of refraction and they possess two corre-
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