• Primary way to observe minerals:
- Grain mounts
- Thin sections

• Tool relies on interaction of minerals and polarized light.

• Important: cheap, quick, easy, only way to determine textures

Light

• Electromagnetic energy derived from excess energy of electrons

• Particle, Wave, or both?
 - Particles = photons
 - For mineralogy, consider light a wave
 - Important wave interference phenomenon

• Light has both electrical and magnetic energy
- Two components vibrate perpendicular to each other
- vibrate perpendicular to direction of propogation
- Electrical component interacts with electrical properties of minerals
 

• Physical description similar to wave:
- v = velocity
- l = wavelength
- f = v/ l
 

• Two light waves that vibrate at an angle to each other:
- vibrations interfere with each other
- interference creates a new wave
- direction determined by vector addition

• Vibration direction of single wave can be split into various components with different vibration directions

• Light composed of many waves:
- wave front = connects same point on adjacent waves
- wave normal = line perpendicular to wave front
- Light ray = direction of propagation of light energy

• A very important point that will come up later:

- in some materials (isometric minerals and other materials – isotropic), wave normals and rays that are parallel
- other materials (non-isometric minerals – anisotropic), wave normals and rays are not parallel.

Polarized light

• Unaltered light from some source (e.g. sun, light bulb) vibrates in all direction perpendicular to direction of propagation

• Possible to polarize light
- passes through anisotropic material
- resolves into two rays, vibrate perpendicularly
- one ray absorbed by material
- light that emerges vibrates in only one direction
 

Interaction of light and matter

• Velocity depends on material light passes through
- vacuum, v = 3.0 * 10¹⁷ nm/sec = 3.0 * 10⁸ m/sec
- all other materials, v <  3.0 * 10¹⁷ nm/sec
• When passes from one material to another
- f = constant
- if v increases, l also must also increase
- if v decreases, l also must also decrease
 

• Isotropic geologic materials
- glass and isometric minerals
- electron density constant in all directions
- no variation with direction of interaction with electromagnetic radiation

• Anisotropic geologic materials:
- minerals in tetragonal, hexagonal, orthorhombic, monoclinic and triclinic systems
- interaction between light and electrons differ depending on direction
 

Reflection and Refraction

• Light crossing boundary of transparent material
 - reflected and refracted

• Reflected light:
 - angle of incidence  = angle of reflection
 - amount controls luster

• Refracted light
- angle of incidence ¹ angle of refraction
- angle of refraction depends on index of refraction
 

• Index of refraction:

 - n = V_v/V_m

V_v = velocity in a vacuum (maximum value)
V_m = velocity in material

- n is always greater than 1
 

• Angle of refraction given by Snell’s law:
 
 
 
 
 
 
 
 

• Snell’s law works for isotropic and anisotropic material if:
- q are angles between normals to boundary
- direction is wave normal and not ray direction
- Important distinction for anisotropic material
 

Critical Angle, CA

• Light going from low to high index material (fast to slow material)
- can always be refracted
- angle of refraction is smaller than angle of incidence

• Light going from high to low index material:
- may not always be refracted
- light is bent toward the interface
- at some critical angle of incidence, the light will travel along the interface
 

• If angle of incidence > CA, then total internal reflection

• CA can be derived from Snell’s law:
 

Dispersion

• Material not always constant index of refraction
• Normal dispersion:
- n higher for short wavelengths (Blue)
- n lower for long wavelengths (red)

• With dispersion, its important to determine n and particular wavelength
- typically n given at l = 486, 589, and 656
- common wavelengths of the sun

• Generally n589 is given.