guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Optical properties: basic principles
4. Optical properties
4.1 Some basic principles
4.1.1 Nature of light, refraction
In order to describe the interaction of light rays with matter, two physical models can be
applied: (a) light as a wave, and (b) light as energy quanta. Most optical phenomena which
are observed during microscopic investigation of amorphous or crystalline substances (glass
phase, minerals), can be adequately explained with the wave model.
Wave model: Light rays propagate as electromagnetic waves. In each wave electric and
magnetic vectors oscillate orthogonal to each other and orthogonal to the propagation
direction. The optical behaviour of light when passing through amorphous or crystalline
substances is essentially controlled by the interaction of the electric vector with the electric
field of the ions. Interactions with the magnetic vector are negligible. Thus, each light wave
can be described as a harmonic oscillation [y = A sin(x)] (Fig. 4-1).
Colour: The human eye can only see a small part of the large spectrum of electromagnetic
radiation, namely the spectral domain between about 400 and 800 nm (visible light). This is
the colour spectrum from violet to blue, green, yellow, orange and red (Fig. 4-1). Sunlight
consists of various proportions of these colours, the combination of which is perceived as
white light. In thin section, colour effects are caused if the spectral composition of originally
white light is changed as light passes minerals, either by depletion of specific wavelengths
(absorption), or by dispersion of white light as a result of refraction or diffraction of light at
grain boundaries, inclusions and rough surfaces.
Intensity: The intensity of light, of a specific colour, for example, is determined by the
amplitude of the light wave. It can be modified by absorption.
Raith, Raase & Reinhardt – February 2012
Polarization: Sunlight or the light emitted from the light source in the microscope consists of
waves which vibrate in random directions. In plane-polarized light, the light waves vibrate in
a defined direction. Plane-polarized light is generated in modern microscopes by a
polarization filter which reduces light of random vibration directions from natural or artificial
sources to light of a single vibration direction (Fig. 4-1). The bundle of light waves entering
the thin section consists entirely of E-W vibrating light waves if the polarizer is adjusted
precisely.
Interference: Two coherent light waves generated by the same light source can overlap (i.e.,
interfere) if they vibrate in the same plane and have the same velocity. This is realised in
optically anisotropic minerals when the two orthogonally vibrating light rays, generated
through double refraction in the crystal plate, are brought to interference in the analyzer after
leaving the thin section (see Ch. 4.2.3). The degree of phase shift (Φ) determines whether the
interfering waves are eliminated or produce a resultant wave of decreased or increased
intensity (Fig. 4-1). If certain sections of the white light spectrum are eliminated, diminished
or amplified, interference colours are generated (see Ch. 4.2.3).
60