|Sample 2nd Hour Exam
|25 March, 1993
347K SECOND HOUR EXAM
Answer the following questions as directed. For multiple choice questions choose the single best answer. Correct answers are in red.
1) Pleochroism in gems is caused by
2) The amount of bending light undergoes when passing through a gem or mineral depends on
a) the angle at which it enters.
3) Dispersion is
a) a phenomenon that produces play-of-color.
4) A polariscope tests for
5) A refractometer can be used to measure
a) refractive index.
6) A dichroscope is used to check for
7) A mineral's critical angle is the angle at which
a) light exiting the mineral is internally reflected.
8) The refractive index of a substance describes
a) how strongly light is bent by the substance.
9) Optically anisotropic minerals differ from isotropic minerals by
a) having low critical angles.
10) Light within a gemstone that strikes a facet at an angle less than the critical angle of the gem will
a) exit the gem.
11) Light that travels through an anisotropic material is always
a) split into 2 rays
12) Light that travels through an isotropic material is always
a) split into 2 rays
13) A gem that looks black every 90o of rotation in a polariscope must be
14) An optic axis is defined as a unique direction in a mineral along which light traveling
a) will be split into two rays
15) A mineral that is trichroic must
a) be isotropic
16) A pleochroic mineral must always be
17) A gem that shows two distinct shadow edges on a refractometer must always be
18) Double refraction is visible in some gems as
19) A gem that is dichroic will
a) have one or two optic axes.
20) A transparent material will only refract light if
a) it is anisotropic
21 The shadow edge of the extraordinary ray in a refractometer can
a) move as the gem is rotated.
22) The flashes of color that are characteristic of a diamonds brilliance are caused by
23) Dull, lifeless, gemstones that show little brilliance are sometimes a consequence of culet angles that are
a) 1 or 2o greater than the critical angle for the gem material.
24) Internal reflection within a gemstone occurs when light strikes a facet at
a) greater than the critical angle
25) Pleochroism is possible in gems of the
a) hexagonal system
Answer the following questions True or False.
26) __T__ Minerals of the isometric (cubic) crystal system are always optically isotropic.
27) __F__ A gem that has a different refractive index depending on how it is oriented on the polariscope must be isometric.
28) __F__ A gemstone with a high critical angle will appear more brilliant than one with a low critical angle, all other factors being equal.
29) __F__ To attain maximum return of light from a gem's pavilion facets, they must be cut so the culet angle is 1 or 2o less than the critical angle for the material.
30) __T__ The ordinary and extraordinary rays in a uniaxial mineral are plane polarized.
31) __F__ The c axis in an isometric (cubic) gem is the direction along which light traveling will not show double refraction.
32) __F__ A refractometer can be used to measure refractive indices of any polished gem or mineral.
33) __T__ A ruby has been faceted so its optic axis is perpendicular to the table facet. When placed table down in a polariscope and rotated about its optic axis, it will remain dark when rotated.
34) __T__ Uniaxial minerals with positive (+) optic signs have extraordinary rays that travel slower than ordinary rays.
35) __T__ Minerals that have high critical angles have low refractive indices.
36) __T__ Minerals that are hexagonal have a single optic axis, have two refractive indices, can be dichroic, and are anisotropic.
37) __T__ Gems of minerals with high R.I.s will be more brilliant than gems of minerals with low R.I.'s, all other factors being equal.
38) __F__ Because both diamond and glass are isometric, an easy way to tell them apart is by testing them in a polariscope.
39) __T__An optically positive mineral must have a birefringence.
40) __F__Light travels faster in gems with higher R.I.s than in gems with lower R.I.s.