guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Optical properties: basic principles
Refraction of light: The velocity of light (as measured in air) is reduced if it enters
substances of higher density (liquids, glasses, minerals). If the angle between the incident
light rays and the phase boundary (e.g., air/glass) is different from 90˚, the light rays change
the propagation direction; they are refracted. Snell's Law of refraction applies (Fig. 4-2) as
long as the materials involved are isotropic (cf. Ch. 4.1.2). In a thin section, the object (d = 25
μm) lies embedded between epoxy resin and glass. As light velocity is almost identical in
glass and epoxy resin, refraction occurs mainly at boundary surfaces between the object and
epoxy resin, but also at phase boundaries within the object (Fig. 4-2).
The light velocity v of a specified material is an important parameter for its identification. For
technical convenience, the refractive index n is used instead of velocity. The refractive index
is defined as the ratio between light velocity v 0 in vacuum (about the same as in air) and light
velocity in the material studied. In isotropic materials, it can be determined experimentally by
measuring the angles of refraction α and β, whereby Snell's Law n 2 /n 1 = sinα/sinβ applies. As
light velocities in all solid and liquid substances are smaller than v 0 (n 1 = n air = 1), refractive
indices are generally larger than 1.
Raith, Raase & Reinhardt – February 2012
Figure 4-2. Refraction of light, dispersion
When passing through a glass prism, rays of
white light are “split up” into their spectral
colours due to differential refraction at the
two prism surfaces. This demonstrates that
ray velocity is dependent on wavelength
(causing dispersion). Therefore, monochromatic
light must be used when determining
refractive indices.
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