Lecture 14 (11/13/2006) Analytical Mineralogy Part 1: Nature of Light Introduction to Optical Mineralogy

Nature of Light  Visible light is a form of electromagnetic radiation, which can be characterized as pulses or waves of electrical energy  Travels in straight lines with a transverse wave motion Unpolarized light Polarized light

Attributes of Light Wavelength (distance between wave peaks; measured in angstroms (Å); defines color of visible light Amplitude (A) height of light waves; corresponds to the intensity/brightness of light Frequency ()  number of light waves passing a fixed point per second; measured in cycles/second Velocity (v = ·); speed of light in a vacuum = 3·10 18 Å/sec = c e.g. for orange light in a vacuum, Å,  = 5·10 14 /sec Light slows down as it passes through denser substances. Because the frequency of light never changes as it passes through different substances, a decrease in light velocity reflects a proportional decrease in its wavelength.

Electromagnetic Spectrum From Bloss, 1961

Reflection and Refraction of Light  When light passes from a low density medium (e.g. air) into a higher density non-opaque medium (e.g. a mineral), part will be reflected and part will be pass through, but be bent and slowed – refracted.  Angle of reflection (  r’ ) equals the incident angle (  i )  Angle of refraction (  r ) will differ from the incident angle depending on the change in velocity between the two substances

Refractive Index and Snell’s Law Index of Refraction – n n substance = c / v substance >1 n substance = c / v substance >1 light velocity in air ≈ c, so n air ~ 1 Snell’s Law- predicts the angle of refraction at the interface of two substances with different refractive indicies: n i sin  i = n r sin  r  r = sin -1 (n i /n r x sin  i )

Critical Angle of Refraction Critical incident angle  i is where  r > 90 º ; i.e. total reflection  i critical = sin -1 (n r /n i x sin 90 º ); e.g., for n i = 2, n r = 1;  i critical = 30 º for n i = 1.5, n r = 1;  i critical = 41.8 º

Successive Refraction

Refraction, Relief, and the Becke Line Relief is the degree to which a phase stands out from its surroundings and is an expression of the contrast in index of refraction dark outline

Becke Line Test From Bloss (1961)

Dispersion  Because n is related to light velocity, which is related to wavelength ((v = ·), different wavelengths of light will have different refraction indicies within a particular substance  Illuminating a mineral with white light may thus lead to color dispersion

Polarization of Light  Light emanating from a point source vibrates in all directions normal to the propagation direction  Light can be polarized (made to vibrate in one plane) by selective absorption (OR) or by reflectance (OL)

Anisotropy Indicies of refraction can vary in all minerals (except those in the isometric system) depending on the orientation of light ray. Such minerals are said to be anisotropic. Isometric minerals, glass, liquids and gasses have a single refraction index value regardless of the orientation of light rays. Such substances are said to be isotropic.

Next Lecture 11/15/06 Optical Properties of Uniaxial Minerals Read: Klein p Perkins and Henke, p