GI-3: Refraction and the Refractometer

Question 1

Refraction

Answer 1

The change in speed and possible bending of light as it passes from one material to another

Question 2

What are two factors that refraction depends on?

Answer 2

1. Optical Density

and

2. Angle of Incidence

Question 3

Angle of Incidence

Answer 3

The angle at which a ray of light strikes a surface, measured from the normal

Question 4

Normal

Answer 4

An imaginary line perpendicular to the point wher ea ray of light strikes the surface

Question 5

Oblique Angle

Answer 5

Any angle other than a right angle

Question 6

Angle of Refraction

Answer 6

The angle between the normal and a refracted light ray.

Question 7

Critical Angle

Answer 7

Angle between the normal and the maximum angle of refraction

Question 8

Refractive Index

Answer 8

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

Question 9

Refractometer

Answer 9

An instrument that measure the critical angel of a gem and translates it directly into RI

Question 10

Polarization

Answer 10

Limiting a light beam to one vibration plane that’s perpendicular to its direction of travel

Question 11

Singly Refractive (isotropic)

Answer 11

Possessing the same physical or optical properties in all crystal directions

Examples:

Diamond, Spinel, Garnet and Amorphous Gems: glass, plastic, and amber

Question 12

Doubly Refractive

Answer 12

Possessing different physical or optical properties in different crystal directions

Question 13

Doubling

Answer 13

The appearance of double images of a gemstone’s features on the side opposite the viewer

Example: Zircon, Peridot

Question 14

Birefringence

Answer 14

The numerical difference between a DR gem’s highest and lowest RIs

Question 15

Optic Axis

Answer 15

Direction of single refraction in a doubly refractive gem

Question 16

Optic Character

Answer 16

The number of optic axes in a doubly refractive gem

Is determined by the stone’s crystal system.

Question 17

Uniaxial

Answer 17

Doubly refractive with one optic axis

Question 18

Biaxial

Answer 18

Double refractive with two optic axes

Question 19

Hemicylinder

Answer 19

Half cylinder in a refractometer, made of high-lead, high-RI glass, on which the stone is placed

Question 20

Monochromatic Light

Answer 20

Light that consists of only one wavelength and one hue

Question 21

Contact Liquid

Answer 21

Liquid that makes an optical contact between a refractometer’s hemicylinder and the material being tested.

Question 22

Over the Limits (OTL)

Answer 22

Description of a gem with an RI higher than that of the contact liquid

Question 23

Parallax

Answer 23

Wavering of RI readings between scale divisions.

Question 24

Spot Reading

Answer 24

Method used to determne RI on a gem’s curved surface.

Question 25

Birefringence Blink

Answer 25

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

Question 26

Optic Sign

Answer 26

Statement of the relationship between a DR stone's two RIs

Question 27

Ordinary Ray

Answer 27

The polarized ray that transmits through a uniaxial stone with an RI that remains constant

Question 28

Extraordinary Ray

Answer 28

The polarized ray that transmits through a uniaxial stone with a varying RI.

Question 29

What are the most accurate curved-surface readings?

Answer 29

50/50 readings

Question 30

What are 3 ways to take a spot RI reading?

Answer 30

1. 50/50 (most accurate) 2. Blink (less accurate) 3. Average (least accurate)

Question 31

What stone give the clearest, most accurate refractometer readings?

Answer 31

Well-polished gems with flat facets

Question 32

When a gem shows definite birefringence on the refractometer, this proves it is

Answer 32

DR-Doubly Refractive

Question 33

The imaginary line perpendicular to the point where a ray of light strikes the surface of a gem is called the

Answer 33

Normal

Question 34

Spectral colors that appear on the refractometer scale might mean that

Answer 34

You need to use less RI liquid

Question 35

If you see a distinct shade cutoff in the refractometer at 1.80, you should

Answer 35

Record the stone's RI as OTL

Question 36

To get a good spot RI reading on a curved surface, how many scale divisions should the spot cover?

Answer 36

2 to 3

Question 37

To check a spot reading for birefringence blink, you must

Answer 37

Remove the magnifier and use white light and the polarizing filter

Question 38

The numerical difference between a DR stones highest and lowest RIs is its

Answer 38

Birefringence

Question 39

When you're testing a well-polished, faceted gem and you use monochromatic light with the refractometer, you will

Answer 39

Get a sharper reading

Question 40

If a gem's high RI remains a constant 1.770 and it low RI varies from 1.762 to 1.770, it's

Answer 40

Uniaxial negative If stones low RI varies more from the common point