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GIA-Colored Stones > GI-3: Refraction and the Refractometer > Flashcards

GI-3: Refraction and the Refractometer Flashcards

1
Q

Refraction

A

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

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2
Q

What are two factors that refraction depends on?

A
  1. Optical Density

and

  1. Angle of Incidence
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3
Q

Angle of Incidence

A

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

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4
Q

Normal

A

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

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5
Q

Oblique Angle

A

Any angle other than a right angle

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6
Q

Angle of Refraction

A

The angle between the normal and a refracted light ray.

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7
Q

Critical Angle

A

Angle between the normal and the maximum angle of refraction

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8
Q

Refractive Index

A

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

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9
Q

Refractometer

A

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

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10
Q

Polarization

A

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

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11
Q

Singly Refractive (isotropic)

A

Possessing the same physical or optical properties in all crystal directions

Examples:

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

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12
Q

Doubly Refractive

A

Possessing different physical or optical properties in different crystal directions

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13
Q

Doubling

A

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

Example: Zircon, Peridot

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14
Q

Birefringence

A

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

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15
Q

Optic Axis

A

Direction of single refraction in a doubly refractive gem

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16
Q

Optic Character

A

The number of optic axes in a doubly refractive gem

Is determined by the stone’s crystal system.

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17
Q

Uniaxial

A

Doubly refractive with one optic axis

18
Q

Biaxial

A

Double refractive with two optic axes

19
Q

Hemicylinder

A

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

20
Q

Monochromatic Light

A

Light that consists of only one wavelength and one hue

21
Q

Contact Liquid

A

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

22
Q

Over the Limits (OTL)

A

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

23
Q

Parallax

A

Wavering of RI readings between scale divisions.

24
Q

Spot Reading

A

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

25
Q

Birefringence Blink

A

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

26
Q

Optic Sign

A

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

27
Q

Ordinary Ray

A

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

28
Q

Extraordinary Ray

A

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

29
Q

What are the most accurate curved-surface readings?

A

50/50 readings

30
Q

What are 3 ways to take a spot RI reading?

A
  1. 50/50 (most accurate)
  2. Blink (less accurate)
  3. Average (least accurate)
31
Q

What stone give the clearest, most accurate refractometer readings?

A

Well-polished gems with flat facets

32
Q

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

A

DR-Doubly Refractive

33
Q

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

A

Normal

34
Q

Spectral colors that appear on the refractometer scale might mean that

A

You need to use less RI liquid

35
Q

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

A

Record the stone’s RI as OTL

36
Q

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

A

2 to 3

37
Q

To check a spot reading for birefringence blink, you must

A

Remove the magnifier and use white light and the polarizing filter

38
Q

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

A

Birefringence

39
Q

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

A

Get a sharper reading

40
Q

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

A

Uniaxial negative

If stones low RI varies more from the common point

GIA-Colored Stones (10 decks)

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