Light and Optics

Question 1

why does light transmitted through a gem slow down?

Answer 1

The electrons inside the gem interact with light much more than those in air.

Question 2

What is the visible light spectrum?

Answer 2

The wavelengths that can be detected by the human eye - between 400nm and 700nm

Question 3

What is white light?

Answer 3

A ray of white light is a combination of all the colours of the range of visible light energy levels. Eg the sun

Question 4

What is reflection?

Answer 4

The return by a surface of all or some of the light which falls on that surface. In gemstones there can be refelction effects from surfaces inside the gemstone such as inclusions and fissures.

Question 5

What is incident light?

Answer 5

Light reaching the surface of a material

Question 6

What is refraction?

Answer 6

The change in speed of light as it enters a gem usually causes its direction to change - this is refraction. The light bends towards the normal.

Question 7

What is the Refractive Index and how is this used in gemmology?

Answer 7

Light travels at different speeds through different gems and this affects the amount of refraction that is experienced. The slower the lights speed in a material the greater the amount of refraction.
The effect varies with the material so it can be used in gem identification

Question 8

What gives us the RI?

Answer 8

A simple relationship between the speed of light in air and in a gem and between the angle if incidence and the angle of refraction.

Question 9

What gives a gem a higher RI?

Answer 9

It will have a higher optical density meaning that it is more densely populated with electrons that interact with light and slow it down more.

Question 10

What is single refraction?

Answer 10

When light travels through a cubic or amorphous material such as a diamond or glass it behaves in the same way regardless of its direction through that material. These materials are OPTICALLY ISOTROPIC they have no directional optical properties.

Question 11

What is double refraction?

Answer 11

This occurs in materials that are OPTICALLY ANISOTROPIC. When light passes through anisotropic materials it is split into two rays as there is less symmetry so rays will travel at different speeds. The two rays will have slightly different refractive indices.

Question 12

What gems is possible to see double refraction in with .a 10x loupe?

Answer 12

Tourmaline, peridot, zircon and synthetic moissanite

Question 13

What is polarised light?

Answer 13

A light ray that has billions of light waves all vibrating in all possible vibration directions. (AKA non polarised light)

Question 14

What is plane polarised light?

Answer 14

Light in which all the waves vibrate in the same direction. It is produced through a polarising filter which is optically anisotropic that removes one of the two vibrating rays.

Question 15

What does plane polarised mean?

Answer 15

The two rays produced when light is transmitted through an anisotropic material are made up of light waves that all vibrate perpendicular to the ray path and the vibration of directions of these two rays are exactly at right angles to each other. The two rays are plane polarised.

Question 16

Explain birefringence

Answer 16

The two rays of light travel at slightly different speeds through most directions in the material thus they have different refractive indices. Birefringence is the maximum numerical difference between the two refractive indices of an optically anisotropic material.

Question 17

What is an optic axis?

Answer 17

The direction of single refraction in an anisotropic material.

Question 18

What are uniaxial stones?

Answer 18

Crystals in the tetragonal, trigonal and hexagonal systems transmit one ray of constant speed (and constant RI) regardless of its vibration direction in the crystal structure They also have one ray of varying speed and RI according to the vibration direction it takes through the crystal.

Question 19

What are biaxial stones?

Answer 19

Orthorhombic, monoclinic and triclinic crystals produce two plane polarised rays which both vary in speed and RI with changing vibration directions. Materials within these systems have two optic axes.

Question 20

What is Total Internal Reflection?

Answer 20

1. Light passing from a material with a high RI into one with a lower RI is refracted away from the normal
2. As the angle of incidence increases the angle of refraction also increases. At a certain angle of incidence the angle of refraction is 90 degrees
3. At this angle of incidence, the critical angle, the light passes along the interface
4. As the angle of incidence increases beyond the critical angle, the ray of light is totally reflected back into the material. this is TOTAL INTERNAL REFLECTION

Question 21

Explain Chatoyancy

Answer 21

AKA cats eye effect, is a bright line or band of light that moves across the surface of some suitably cut gemstones.
It is produced from many long parallel inclusions
It is a reflection from crystal needles or fibres from tube like cavities within the gemstone.

Question 22

Explain asterism

Answer 22

Asterism is a star like reflection effect

It is produced from long and thin inclusions that are abundant and parallel in at least two directions

Question 23

What causes the colour and glittering effect in aventurine quartz?

Answer 23

spangles of mica

Question 24

Whats causes the golden spangles and iridescence in Sunstone

Answer 24

platelets or needles of hematite

Question 25

What is Brilliance?

Answer 25

The degree of brightness or light return resulting from reflection of light from a faceted gemstone when viewed face up. It is both an internal and external reflection effect.

Question 26

What is a polariscope?

Answer 26

It has two polarising filters that are set in a crossed position so that the transmitted vibration direction of polarisation of the upper filter is at right angles to the lower filter

Question 27

If a stone remains dark through 360 what can you conclude?

Answer 27

Its optically ISOTROPIC, either amorphous or cubic eg garnet, spinel, diamond, fluorite, paste, opal

Question 28

If a stone goes light and dark through 360 four times what can you conclude?

Answer 28

It is optically ANISOTROPIC, either uniaxial or biaxial

Question 29

If a stone stays light through 360 what can you conclude?

Answer 29

It is POLYCRYSTALLINE

Question 30

What is the anomalous extinction effect?

Answer 30

The material is under internal stress. Stress tensions and pressures within a material cause strain. With strain electronic bonds line up in certain directions. This shows up as certain amount of optical anisotropy within an otherwise isotropic material.

Question 31

What stone shows tabby extinction effect?

Answer 31

Verneuil synthetic spinel. look for thin ‘clouds’

Question 32

What is monochromatic light?

Answer 32

‘one colour’ of a sodium yellow light of 589nm wavelength. most gemmologists use a special monochromatic filter or LED

Question 33

What does the refractometer do?

Answer 33

light shone through tabe onto stone. some passes through stone, some reflected back into refractometer
the reflected light partly illuminates a scale in the refractometer
the boundary between the light and shadowed parts of the scale indicate RI

Question 34

What is the RI of contact fluid?

Answer 34

1.79 - the highest RI that is possible to read on refractometer

Question 35

What would you expect to see on a refractometer with an amorphous or cubic stone?

Answer 35

One stationary shadow edge

Question 36

What would you expect to see on a refractometer with an anisotropic uniaxial stone?

Answer 36

One stationary shadow edge

One may move

Question 37

What would you expect to see on a refractometer with an anisotropic biaxial stone?

Answer 37

Both shadow edges may move