GI-3: Refraction and the Refractometer Flashcards
Refraction
The change in speed and possible bending of light as it passes from one material to another
What are two factors that refraction depends on?
- Optical Density
and
- Angle of Incidence
Angle of Incidence
The angle at which a ray of light strikes a surface, measured from the normal
Normal
An imaginary line perpendicular to the point wher ea ray of light strikes the surface
Oblique Angle
Any angle other than a right angle
Angle of Refraction
The angle between the normal and a refracted light ray.
Critical Angle
Angle between the normal and the maximum angle of refraction
Refractive Index
A measure of the change in the speed and angle of light as it passes from material to another
The smaller a gem’s critical angle, the higher it’s RI
Refractometer
An instrument that measure the critical angel of a gem and translates it directly into RI
Polarization
Limiting a light beam to one vibration plane that’s perpendicular to its direction of travel
Singly Refractive (isotropic)
Possessing the same physical or optical properties in all crystal directions
Examples:
Diamond, Spinel, Garnet and Amorphous Gems: glass, plastic, and amber
Doubly Refractive
Possessing different physical or optical properties in different crystal directions
Doubling
The appearance of double images of a gemstone’s features on the side opposite the viewer
Example: Zircon, Peridot
Birefringence
The numerical difference between a DR gem’s highest and lowest RIs
Optic Axis
Direction of single refraction in a doubly refractive gem
Optic Character
The number of optic axes in a doubly refractive gem
Is determined by the stone’s crystal system.
Uniaxial
Doubly refractive with one optic axis
Biaxial
Double refractive with two optic axes
Hemicylinder
Half cylinder in a refractometer, made of high-lead, high-RI glass, on which the stone is placed
Monochromatic Light
Light that consists of only one wavelength and one hue
Contact Liquid
Liquid that makes an optical contact between a refractometer’s hemicylinder and the material being tested.
Over the Limits (OTL)
Description of a gem with an RI higher than that of the contact liquid
Parallax
Wavering of RI readings between scale divisions.
Spot Reading
Method used to determne RI on a gem’s curved surface.
Birefringence Blink
A method of determining birefringence that involes look for a light-to-dark or green-to-red change in a spot.
Usually occurs with cabonates such as Calcite, marble, malachite, and rhodoschrosite
Optic Sign
Statement of the relationship between a DR stone’s two RIs
Ordinary Ray
The polarized ray that transmits through a uniaxial stone with an RI that remains constant
Extraordinary Ray
The polarized ray that transmits through a uniaxial stone with a varying RI.
What are the most accurate curved-surface readings?
50/50 readings
What are 3 ways to take a spot RI reading?
- 50/50 (most accurate)
- Blink (less accurate)
- Average (least accurate)
What stone give the clearest, most accurate refractometer readings?
Well-polished gems with flat facets
When a gem shows definite birefringence on the refractometer, this proves it is
DR-Doubly Refractive
The imaginary line perpendicular to the point where a ray of light strikes the surface of a gem is called the
Normal
Spectral colors that appear on the refractometer scale might mean that
You need to use less RI liquid
If you see a distinct shade cutoff in the refractometer at 1.80, you should
Record the stone’s RI as OTL
To get a good spot RI reading on a curved surface, how many scale divisions should the spot cover?
2 to 3
To check a spot reading for birefringence blink, you must
Remove the magnifier and use white light and the polarizing filter
The numerical difference between a DR stones highest and lowest RIs is its
Birefringence
When you’re testing a well-polished, faceted gem and you use monochromatic light with the refractometer, you will
Get a sharper reading
If a gem’s high RI remains a constant 1.770 and it low RI varies from 1.762 to 1.770, it’s
Uniaxial negative
If stones low RI varies more from the common point
