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

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 = 3·1018 Å/sec = c
e.g. for orange light in a vacuum,  = 6000Å,  = 5·1014 /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
nsubstance = c / vsubstance
>1
light velocity in air ≈ c, so nair ~ 1
Snell’s Law- predicts the angle of refraction
at the interface of two substances with
different refractive indicies:
ni sin i = nr sin r
r = sin-1 (ni/nr x sin i)
Critical Angle of Refraction
Critical incident angle i is where r > 90º; i.e. total
reflection
icritical = sin-1 (nr/ni x sin 90º);
e.g., for ni = 2, nr = 1; icritical = 30º
for ni = 1.5, nr = 1; icritical = 41.8º
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. 298-303
Perkins and Henke, p. 14-22