Becke Lines, Dispersion

Question 1

The refractive index of a cubic (system) crystal or a homogeneous non-crystalline material (such
as glass or a fluid) does not vary with the direction of light propagation. Therefore, these
substances are ____ with respect to the propagation of light.

Answer 1

isotropic

Question 2

T or F. Becke lines are most
easily seen with the medium power objective of a microscope and with the substage diaphram
mostly closed. The greater the difference in refractive index between the oil and the mineral, the
brighter the Becke line and the greater the relief of the mineral.

Answer 2

True

Question 3

It is a qualitative measure
of the contrast (shadows and Becke lines) between the mineral and the oil caused by refraction
and reflection at their interface.

Answer 3

Relief

Question 4

T or F. When the mineral and oil have the same refractive index, the
mineral will be almost invisible in the oil (very low relief) and the Becke line(s) will be faint and
colored.

Answer 4

True

Question 5

If the microscope stage is lowered, the Becke line will appear to move toward the medium
(mineral or oil) that has the ____ (higher or lower) refractive index.

Answer 5

higher

Question 6

Shadows and bright areas due to _____ may also be used to determine relative refractive indices.

Answer 6

oblique illumination

Question 7

T or F. The refractive index of
the isometric mineral is determined by repeated tests using different oils until a refractive index
match is obtained.
mineral
grain
mineral
grain
oil.

Answer 7

True

Question 8

The refractive index of most materials varies with the wavelength of light, a property called _____.

Answer 8

Dispersion

Question 9

The index of refraction is higher for ____ wavelengths of light and decreases monotonically with _____ wavelength.

Answer 9

Shorter; increasing