Chapter 9Optical Properties

Objectives Understand principles of:

• Refraction of light• Refractive indexes• Polarization of light• Birefringence• Pleochroism• Optical indicatrix• Dispersion

The importance of optical properties of minerals

Optical mineralogy:• study of interaction of polarized light in minerals• NB for:

ID minerals Reveal characteristics of minerals

Petrographic mineralogy:• Systematic description of minerals as they occur in rocks

in THIN SECTION Basis of petrographic mineralogy:

• Isotropy and anisotropy Isotropy: Homogeneity in all directions Anisotropy: presence of a preferred orientation

• Preferred orientation: concentration of linear or planar, structural or fabric elements with a preferred attitude

• Directional interaction of light with crystals

Isotropic/Anisotropic Isotropic Isometric

Anisotropic Uniaxial TetragonalHexagonalTrigonal

Anisotropic BiaxialOrthorombic

MonoclinicTriclinic

Waves and light Visible light is electromagnetic radiation (Fig 9.1)

• Electric field E creates magnetic field H (right angles to E)

• Propagate at velocity c in vacuum

Light: particle (photon) and wave properties; move in straight direction @ velocity c• Use either property depending on optical effect required

All visible wavelengths together (400-800nm) – white light

Monochromatic light – one wavelength (sodium vapor lamp)

Spectrum of electromagnetic radiation

Refraction Change in direction of a wave due to

a change in velocity Occur when wave passes between

different mediums Most common example: refraction of

light• Also in sound waves, water waves

Refraction of light

Refraction Described by Snell’s law:

• The angle of incidence and the angle of refraction are related to the velocity of the incidence and refracted rays and inversely related to the refractive index of the two mediums of travel

• In other words: When light goes from less dense to denser

medium, it changes direction – refracted• Except for 90º incidence angle

Snell’s law

READ: Box 9.1 in textbook for more detailed explanation

Refractive Index A measure for how much the speed of

waves is reduced inside a medium compared to inside a vacuum• n = c/v the velocity ratio

c = velocity of light in vacuum (vvacuum) v = velocity of light in medium (vmedium)

• In mineralogy the medium is a mineral (vmineral)• Thus: nmin = vvacuum / vmineral

Refractive index Light in mineral: scattered by electrons – time

delay observed• Thus: fundamental velocity of the light wave does not

change – in reality the denser electron-packing causes a longer path length for the wave

Refractive index increase when number of electrons per unit volume increase in general when density increase

RI can vary with:• direction of light• wavelength• temperature

Determining the refractive index of a mineral

Used in optical microscopic mineral ID• Minerals has characteristic refractive indices

• Make use of liquids with known refractive indices to determine the index of the mineral in question

• Determine by means of relief and Becke Line

Determining the refractive index of a mineral

Determining the refractive index of a mineral

Determining the refractive index of a mineral

Polarization Polarization - describes orientation of oscillations

of waves• Transverse waves (light)

Oscillations in plane perpendicular to direction of propagation

Polarization When a wave of light is filtered to

have only one vibrational direction Vibrations in plane perpendicular to

propagation

Under microscope:• Observed as:

Pleochroism – plane polarized light Birefringence – crossed polarized light

Obtaining polarization By using polaroid filter

• Organic synthetic crystals• The most common method of polarization

Shows strong preferential absorption due to different bonding forces in different crystal directions

• Made of material capable of blocking one of two planes of vibration of electromagnetic wave

Thus: filters out one-half of vibrations

• Unpolarized light into Polaroid filter emerges: half intensity; single plane vibrations polarized light

• Other crystals; many minerals also show directional absorption not complete absorption - only certain wavelengths Colour changes during rotation in polarized light

PLEOCHROISM – very useful ID tool

Also obtain polarization by:• Reflection• Refraction• Scattering

Birefringence When a ray of light is split into two

separate polarized rays – each with a single vibration direction perpendicular to that of the other ray

Under the microscope:• Observed under crossed polarized light

as: Interference colors Only in anisotropic minerals

Birefringence/double refraction Decomposition of a ray of light into two rays (the

ordinary ray, ω, and the extraordinary ray, ε) when passing through certain types of material, such as calcite crystals or boron nitride

Only in anisotropic minerals Uniaxial birefringence: material with two different

refractive indices – nω and nε Biaxial birefringence (trirefringence): material

with refractive index of three “equal” values - nα, nβ and nγ

Calculated as the path difference between polarized rays after leaving a crystal of thickness d – also called optical retardation

Birefringence• Incident light refracted into two different paths

- split into two beams• Show two images if object is viewed through

double refractive crystal• Result of BIREFRINGENCE of light – both beam

polarized, but perpendicular to each other Due to different refractive indices of mineral in the

two or three different directions• Two rays can be blocked out individually by

filters Evidence for the wavelike behaviour of light

Birefringence

Birefringence/double refraction Doubly refracted waves are polarized but

separate, vibrating in different planes – no interaction

Need interference to study interference colours and other properties• To get interference – a second polarizer

inserted – the analyzer: Crossed polarizer

• Used to analyze the interference effects of light in minerals

NB Terminology Optical mineralogy Anisotropy Polarized light Crystal Mineral Petrographic mineralogy Preferred orientation Refraction of light Refractive index Birefringence Ordinary ray Extraordinary ray Thin section

Pleochroism Uniaxial Biaxial Interference colors Monochromatic light Relief Becke line Plane polarized light Crossed polarized light Path difference Optical retardation Analyzer Optical plane Optical axis