Pleochroism in gems is caused by diffraction.
dispersion.
absorption of different wavelengths of light in different directions.
the presence of more than one chromophore.
the absence of an optic axis.
The amount of bending light undergoes when passing through a gem or mineral depends
on
a) the angle at which it enters.
b) the refractive index of the gem or mineral.
c) the wavelength (color) of the light.
d) all of the above.
e) b and c.
Dispersion is
a) a phenomenon that produces play-of-color.
b) an optical property that can be measured with a refractometer.
c) related to the difference in the speed of red and blue light in a gem or mineral.
d) something that only occurs in anisotropic materials.
e) b and c
A polariscope tests for
dispersion.
diffraction.
refractive index.
the presence of chromophores.
none of the above
A refractometer can be used to measure
a) refractive index.
b) birefringence.
c) optic sign.
d) pleochroism.
e) a, b and c
A dichroscope is used to check for
play-of-color
pleochroism
asterism
dispersion
birefringence
A mineral's critical angle is the angle at which
light exiting the mineral is internally reflected.
light entering the mineral is refracted to 40o.
light exiting the mineral is refracted parallel to the surface of the mineral.
light exiting the mineral is refracted away from the normal.
none of the above.
The refractive index of a substance describes
how strongly light is bent by the substance.
whether a substance can polarize light.
whether a substance can split light into two plane polarized rays.
the amount of diffraction occurring within a substance.
all of the above.
Optically anisotropic minerals differ from isotropic minerals by
having low critical angles.
being able to polarize light.
having high critical angles.
being fluorescent in ultraviolet light.
none of the above.
Light within a gemstone that strikes a facet at an angle less than
the critical angle of the gem will
exit the gem.
be internally reflected.
be refracted parallel to the facet.
be split into two plane polarized rays.
none of the above
Light that travels through an anisotropic material is always
split into 2 rays
polarized
doubly refracted
diffracted
none of the above
Light that travels through an isotropic material is always
split into 2 rays
polarized
doubly refracted
diffracted
none of the above
A gem that looks black every 90o of rotation in a polariscope must be
anisotropic
isotropic
monoclinic or triclinic
hexagonal or tetragonal
none of the above
An optic axis is defined as a unique direction in a mineral along which
light
will be split into two rays
will be polarized into two directions
will pass through without being split or polarized
will be most strongly absorbed
none of the above
A mineral that is trichroic must
be isotropic
be isometric (cubic).
be triclinic.
in some orientations blink from dark to light when rotated in a polariscope.
none of the above.
A pleochroic mineral must always be
isotropic.
anisotropic.
hexagonal.
tetragonal.
none of the above.
A gem that shows two distinct shadow edges on a refractometer must always be
pleochroic.
isotropic.
anisotropic.
uniaxial.
none of the above
Double refraction is visible in some gems as
dispersion.
a doubling of pavilion facet junctions when viewed through the table.
oriented inclusions.
total internal reflection
none of the above
A gem that is dichroic will
have one or two optic axes.
have more than one refractive index.
be anisotropic.
be a member of either the hexagonal, tetragonal, monoclinic, triclinic or orthorhombic
crystal system.
all of the above.
A transparent material will only refract light if
it is anisotropic
it has a birefringence
it can polarize light
the light enters at greater than 0o to the normal
all of the above
The shadow edge of the extraordinary ray in a refractometer can
move as the gem is rotated.
be less than the shadow edge of the ordinary ray.
be greater than the shadow edge of the ordinary ray.
overlap the shadow edge of the ordinary ray if the birefringence is low.
all of the above
The flashes of color that are characteristic of a diamonds brilliance are caused by
birefringence
anisotropism
dispersion
double refraction
total internal reflection
Dull, lifeless, gemstones that show little brilliance are sometimes a consequence
of culet angles that are
1 or 2o greater than the critical angle for the gem material.
less than the critical angle for the gem material.
equal to the normal.
less than the reflective angle for incoming light through the table.
too shallow for a properly cut crown.
Internal reflection within a gemstone occurs when light strikes a facet at
greater than the critical angle
less than the critical angle
the critical angle
an angle parallel to the normal
parallel to the optic axis
Pleochroism is possible in gems of the
hexagonal system
triclinic system
monoclinic system
tetragonal system
all of the above