Chapter 6 Light and Optics

What is Light

Light is a form of energy.

Light interacts with the energy of electrons in the atoms of a gem

Light is a wave motion. What kind of wave motions are there?

the direction of propagation

the vibration direction

what is the direction of propagation?

the direction taken by a single wave of light

What is the vibration direction?

The direction in which a single wave of light vibrates as it travels

What is the Spectrum

The spectrum is electromagnetic energy = visible light.

The wavelength is between 400 - 700nm

The spectrum that a human eye can see.

what are optical properties?

Interactions between different materials with visible light energy that humans can observe and measure.

what optical properties are there

Reflection

Refraction

What is reflection?

How light bounces off a surface.

The return by a surface of all or some of the light which falls on that surface

what is refraction?

refraction can happen when light passes into a material.

the change of the speed of light as it enters the gem (usually) causes its direction to change or bend.

Why can you see transparent gems

Because the bending effect of refraction makes them more visible. The slower the speed of light, the greater the bending effect.

What is RI

The Refractive Index measures optical density.

Higher RI materials are more densely populated with electrons, which interact with light and slow it down more.

WHAT IS SR

SR = Single Refraction

what is optically isotropic

The same all-round/ No directional properties.

When light travels through a sample of amorphous material, it behaves the same way, regardless of its direction through that material.

Which material is SR

Amorphous and Cubic system

Cubic and Amorphous materials

no directional properties

They have physical directional properties: hardness and cleavage

The rest of the crystal system…

directional optical properties AND physical directional properties

what is DR

Double refraction = optical doubling which is caused by the splitting of light into two separate rays.

by an optically anisoptropic material

may be seen with a 10x loupe, microscope or sometimes with unaided eyes.

“space” for AI, “/” for format.

what is optically anisotropic

Not the same all round/directional optical properties

when light travels through an optically anisotropic crystalline material, it is forced by the anisotropic crystal structure to vibrate in two distinct components.

the two rays are refracted differently within the same material

Why are the two rays refracted differently within the same material

The two rays travel at slightly different speeds through most directions in the material

DR and optically anisotropic and uniaxial?

Trigonal, Tetragonal and Hexagonal

DR and optically anisotropic and biaxial?

Orthorhombic, Monoclinic and Triclinic

SR and optically isotropic

Cubic and amorphous material

In which gems can be seen through a 10xlens a clear doubling of inclusions of faceted edges, scratches and dust specks

Tourmaline

Peridot

Zircon

Synthetic moissanite

What is non-polarized light?

Light from the sun or hand lamp.

billions of light waves vibrating in all possible directions=no order to the vibration direction

What is polarized light?

When two rays vibrate perpendicular to the ray path and the vibration directions of the two rays are exactly at right angles to each other.

PPL

Plane Polarized Light (PPL), which is all vibrating in one direction, is produced through a polarizing filter, which is optically anisotropic material that removes one of the component rays.

What is birefringence?

The maximal numerical difference between two refractive indices of an optically anisotropic material is measured with a refractometer.

Every optically anisotropic material has a max and min RI

Tetragonal, Triginbal and hexgonal; birefringence?

one constant ray

one speed

single, unvarying Refractive Index (RI)

orthorhombic, Monoclinic Triclinic

both of the 2 plane polarized rays vary in speed with changing directions

Optically, anisotropic material has two options

1. One direction parallel to which no double refraction occurs=one optical axes=optically uniaxial

2. Two directions parallel to which no double refraction occurs= two optic axes=optically biaxial

This polarizing effect is never seen in amorphous or cubic system. why not?

These structures do not posses directional optical properties

In which materials will DR not be seen?

Glass, Spinel, Garnet, Fluorite and Diamond

What is TIR?

Total Internal Reflection

The ray of light is totally reflected back into the material

this happens at a critical angle= the light passes along the surface itself .

in that case, the angle of refraction = 90 degrees.

when the angles of incident increase, the light reflects totally back into the material

when is TIR mentioned

with brilliance and refractometer

Reflection effects caused by inclusions

Asterism and Chatoyancy

what is chatoyancy?

• multiple internal reflection effects

• appears as a single bright line or band of light that moves across the surface of certain well cut gemstones

• effect is from many long parallel inclusions

When is chatoyancy shown?

• at the right angle of the inclusions.

• inclusions are long and thin

• inclusion are in parallel arrangement

• inclusions are sufficiently abundant

• gemstone is cut in such a way that the effect is revealed

• polished rounded surface (cabochon)

Which gemstones show frequent asterism?

Chrysoberyl, quartz, and tourmaline

glass cabochon

beads made of parallel optic fibres

What is asterism?

• same as chatoyancy

  • multiple internal reflection effects

  • the effect is from many long parallel inclusions

  • However, with the star-like effect, the reflection effect is caused by 2 or more oriented sets of parallel inclusions within a gemstone.

When is asterism shown?

• In a parallel arrangement in at least 2 different directions.

• inclusions needles or fibres/mineral substances or tube like cavities

• inclusions need to be long and thin

• inclusions are sufficiently abundant

• gemstone is cut in such a way that the effect is revealed

• polished rounded surface (cabochon)

• star cabochon is cut so that its base is parallel to all of the inclusions.

• each individual arm of the star is a band of light at 90 degrees to the direction of the inclusions causing it,

which gemstones show asterism frequently

Corundum (Ruby and Sapphire), almandine garnet and Quartz

LET op:

• natural start rubies/sapphires: often deeply domed base

• synthetic: usually flat-based

other internal reflection effects

• golden spangles + iridescence

• Rainbow + reflection effect

• glittering effect with spangles of mica or copper crystals

golden spangles?

platelets or needles of hematite and other minerals

VB. Sunstone = a variety of feldspar

rainbow effect?

microscopic crystals and highly elongated gas bubbles

VB Obsidian = volcanic glass

glittering effect

Spangles of mica= stone with vitreous to pearly lustre

VB aventurine quartz

spangles of copper crystals

VB. Aventurine glass (goldstone)

what is brilliance?

A degree of brightness (return of light) results from the reflection of light by a faceted gemstone when viewed ‘face-up.’

It is an internal-external reflection effect.

The amount of maximum Brilliance depends on the following

• degree of transparency

• degree of calrity (freedom of inclusiosn)

• good polish

• high lustre

• freedom form wear

vb of brilliance

Diamond vs. Cubic Zirconia (CZ): A black line on paper: Look through the table of the gem down. The diamond is not visible as the light is reflected back. With CS, the line is visible.

WHY? The lower RI of the CZ cutting angle is too low for much TIR to occur. Optimum brilliance for CZ requires very steep pavillion angles, which are too deep for normal settings.

What does a polariscope

• Test transparent + some translucent gem material = light needs to go through.

• can be used for gem material in almost any form (rough, faceted, carved)

• set stones, strung beads, several unset stones at once

how does a polariscope works?

• 2 polarizing filters : set in a crossed position so the transmitting vibration direction of polarization of the upper filter is a right (90 degrees) degrees angle to the lower filter.

• place over light source

• view=black

• rotate

what are Anomalous extinction effects

Internal stress is causing strains, and as a result, the electric bonds tend to line up in certain directions

the result is a certain amount of optical anisotropy within an otherwise isotropic material

examples of anomalous extinction effects

• In many glass stones (paste0

• in striped banding almandine garnet

• as bands/zones in diamond

• as motted/zoned effects in plastics

• on amber, particularly ‘pressed amber’

• in synthetic spinel grown by verneuil method= tabby extinction = stripes of a tabby cat.

wat is anisotropy

the property of being directionally dependent

wat is isotropy

homogeneity in all directions

uniform in all directions

isos=equal and tropos = way

what does a refractometer

• makes use of the TIR to provide the value for the Refractive Index (RI)

• uses monochromatic light = one colour means that a sharp edge is obtained

• use of white light would give a rainbow of colours=not possible to read.

• The reflected light illuminates a scale. The boundary between the reflected light and partially reflected light appears as a shadow edge, which is the RI of the gem.

how to use the refractometer

• Dark scale; RI of gem is too high to read =

diamond, zircon, spessartine garnet, andradite garnet

• Single shadow = SR

• Double shadow = DR

• Porous material can be stained by the fluid (coral, pearl, bone, turquoise and malachite, ivory, opal)

• damaged by the fluid: amber, plastic and composed materila

• Contact fluid is approx, 1.79-1.81

what is shown at the refractometer:

Optically uniaxial = one moves during the rotation of 360 degrees two shadows

Optically biaxial = both shadow edges move during rotation 360 degrees