EARTH MATERIALS OPTICS AND MINERALS Wave Terms FREQUENCY (f) - - PowerPoint PPT Presentation

EARTH MATERIALS OPTICS AND MINERALS

FREQUENCY (f) number of cycles per unit time [units = Hertz (Hz)] 1 Hz = 1 cycle/s T = 1/f; f = 1/T; T f = 1 Wave Terms

Waves can coexist in the same space with other waves. Waves interact in various ways. Waves that are in phase experience constructive interference.

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waves that are out of phase experience destructive interference...

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Objects radiate light because of their high temperatures - incandescent incandescent. light (EM) is produced whenever a charge is accelerated by an external force... electromagnetic wave model

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Visible light – part of the EM spectrum Longer v = f, and v = c

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Peak solar %

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Waves may be

• Absorbed
• Transmitted

Refracted

• Reflected

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v c n =

Index of refraction (n) for EM

air (0oC) 1.00029 air (30oC) 1.00026 Glass 1.50 Diamond 2.42 Ice 1.31 Water 1.33 Alcohol 1.36

Refraction

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“You know how a stick looks bent when you put it in water? That’s why I no longer take baths.” Steven Wright

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b a A B r i

n n v v = =

• sin

sin

Snell’s law

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Critical Angle All you need to find this is to solve Snell’s law for r =90O

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Polarization Polarization

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Plane Polarization Filter

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When monochromatic light is transmitted through a mineral from air, its velocity slows, the waves bunch up. The frequency remains the same but the wavelength decreases.

Refraction – producing constituent wavelengths

Dispersion

High f (low ) – more bending in prism

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The index of refraction decreases with increasing

• wavelength. The nD is

at 589 nm. Abnormal dispersion

• f the ri (increased at

higher ). Minerals with abnormal dispersion are always colored.

Immersion oils of fixed ri can identify the ri of an unknown grain. If the mineral has a higher ri = positive relief If the oil has a higher ri = negative relief Lowering the stage of a microscope, the focus (Becke) line moves to the material with higher ri.

Isotropic

Same in every

• direction. Light is

refracted equally in all directions in these materials. Isometric forms (cubic) Non-lattice structures

Glasses, polymers, liquids, gasses.

ri is uniform

Uniaxial

Two principle directions at right angles. Tetragonal Hexagonal Rhomobohedral 2 configurations If c > a then (+) If c < a then (-) Two ri

• Those with atoms equally spaced along two axes,

elongated in the third.

• Requires two raypaths, one in the close packed

plane, one perpendicular. The ordinary () ray and the extraordinary () ray

• If the extraordinary ray is the slow ray, then the

mineral is optically positive

• If the extraordinary ray is the fast ray, then the

mineral is optically negative.

Uniaxial

Anisotropy results when atoms are closely packed in one plane, and further apart in another. In anisotropic materials, the velocity of light varies depending on direction through the material In most directions, light that enters splits into two rays that vibrate and right angles to each other. d = thickness = retardation

Interference at the analyzer

The retardation is a full wavelength - the fast and slow rays constructively interfere perpendicular to the analyzer The retardation is a half wavelength - the fast and slow rays constructively interfere in the pole of the analyzer

Michel-Levy Interference Color Chart

Nikon Microscopes

Color Interference (birefringence)

Uniaxial

Uniaxial

Interference figures Fast on slow -colors go low

Biaxial crystals

Three principle directions, not necessarily at right angles. Orthorhombic Monoclinic Triclinic

• Those with atoms unequally spaced along the three

crystallographic axes.

• Requires three raypaths for each direction

The , , rays is lowest ri, fastest is highest ri, slowest is the middle child

• This produces two optical axes, separated by an

acute angle 2Va.

• If 2Va is bisected by n, then positive.
• If 2Va is bisected by n, then negative

Biaxial crystals

Biaxial

http://www.brocku.ca/earthsciences/people/gfinn/optical/bxsign1.htm

http://edafologia.ugr.es/optmine/xplconos/fbtallaw.htm

Minerals retard light relative to air Light is refracted as a function of the velocity difference. Polarization eliminates all but one vibration direction Isotropic materials have only one velocity - no interference Uniaxial minerals have two raypath velocities -

• ptic axis is on c and exhibit interference

Biaxial minerals have three raypath velocities - two

• ptic axes and exhibit interference