Optical Mineralogy Use of the petrographic microscope © John Winter, Whitman College with some slides © Jane Selverstone, University of New Mexico, 2003

Why use the microscope?? Identify minerals (no guessing!) Determine rock type Determine crystallization sequence Document deformation history Observe frozen-in reactions Constrain P-T history Note weathering/alteration © Jane Selverstone, University of New Mexico, 2003

The petrographic microscope Also called a polarizing microscope physics of lighttools and tricks In order to use the scope, we need to understand a little about the physics of light, and then learn some tools and tricks… © Jane Selverstone, University of New Mexico, 2003

What is light? A wave or a particle (photon)? Wave theory: light travels wavelike from point A to point B with electrical and magnetic properties (electromagnetic radiation)

Continuous spectrum of radiation

Electromagnetic spectrum Electromagnetic spectrum & visible portion Violet (400 nm)  Red (700 nm) White = ROYGBV dispersion (can be separated by dispersion in a prism)

Electric and magnetic components to light Vibrate perp. to each other perp. To direction of propagation Ignore magnetic compnent

Light has: velocity Wavelength ( )

Light has: velocity Wavelength ( ) Frequency F = V/

Frequency is constant Change wavelength, must change velocity

Wave: single pulse of energy advancing Wave front: surface through all similar points on adjacent waves

Wave normal: line perpendicular to wave front = direction wave is traveling

Light ray: direction of light energy propagation Doesn’t have to = wave normal

Isotropic minerals: wave normal and light rays are perpenduclar to wave front Anisotropic minerals: wave normal and light ray directions not parallel

Two wave vibrating in same plane, traveling on same path will interfere Retardation (  ): distance that one wave lags behind the other

In phase If peaks and wavelengths correspond

Out of phase Completely out of phase cancel out

Partially out of phase

What happens as light moves through the scope? light source your eye light ray waves travel from source to eye wavelength, I = f(A) amplitude, A light travels as waves Frequency = # of waves/sec to pass a given point (hz) f = v/ v = velocity (in physics v = c, but no longer) © Jane Selverstone, University of New Mexico, 2003

When light passes from one material to another; can be reflected Angle of incidence = angle of reflection

When light passes from one material to another; can be refracted Index of refraction: how bent the light is passing through a material High index = low velocity

Light from the sun: Vibrates in all directions, perp. To direction of propagation Unpolarized light

Light in microscope: Polarized light Vibration constrained to one direction

Effect of polarization: Polarized light Vibration constrained to one direction

Light beam = numerous photons, each vibrating in a different plane Vibration in all directions ~ perpendicular to propagation direction vibration directions propagation direction What happens as light moves through the scope? Polarized LightUnpolarized Light Each photon vibrates as a wave form in a single plane Light beam = numerous photons, all vibrating in the same plane planes of vibration

How to polarize light: Polarizer sheets

1) Light passes through the lower polarizer west (left) east (right) Plane polarized light “PPL” Unpolarized light © Jane Selverstone, University of New Mexico, 2003 light intensity decreases Only the component of light vibrating in E-W direction can pass through lower polarizer – light intensity decreases

upper polarizer 2) Insert the upper polarizer west (left) east (right) Now what happens? What reaches your eye? Why would anyone design a microscope that prevents light from reaching your eye??? XPL=crossed nicols (crossed polars) south (front) north (back) Black!! (“extinct”) © Jane Selverstone, University of New Mexico, 2003

Crossed-polars

Other ways to polarize light Hit light on smooth surface (light, glass)

Other ways to polarize light Brewster’s Angle: angle of incidence needed to polarize reflected light Need 90° between reflected and refracted rays n 2 /n = Tan (Brewster’s angle)

Brewster’s angle is reason polarized sunglasses work so well Ever turn your head 90° wearing polarized sunglasses?