Page 215 - A Practical Introduction to Optical Mineralogy

INTRODUCTION
5 Reflected-light theory (a) Line (b) Circle

5.1 Introduction

The nature of polarised light is described in Section 4.1, which should be
referred to if the reader is uncertain about what is meant by 'polarised'
light.
In order to understand the optical properties of minerals in reflected
light it is necessary to consider elliptically polarised light as well as
linearly (or plane) polarised light. The concept of polarisation of light is
discussed in detail in Galopin and Henry (1972), but the brief simplified
and idealised account presented here should be adequate for beginners. (c) Ellipse
The three categories of polarised monochromatic light are illustrated in
Figure 5.1 and are named according to the nature of the cross section of
the wave when viewed along the path of the ray. Vibration of a particle
up and down to produce a wave confined to a plane is easy to visualise,
but this is not true of vibration leading to ellipticity. Elliptically polarised
light may be considered to consist of two linearly polarised components
which are out of phase and vibrate at right angles. Elliptically polarised
light can only be partially extinguished by rotating a polariser in its path,
whereas linearly polarised light is completely extinguished when its
vibration direction is normal to that ofthe polariser. Circularly polarised
light is a special case of elliptically polarised light where the two compo-
nents have the same amplitude and a path difference of one-quarter or
Figure 5.1 Three categories of polarised monochromatic light.
three-quarters of a wavelength.
In reflected light microscopy we are dealing with normally incident
5.1.1 Reflectance
linearly polarised white light, but the light reflected from the polished
surface only remains lineary polarised in certain cases; all sections of The brightness of a mineral observed using reflected light microscopy
cubic minerals and some sections of non-cubic minerals in certain orien- depends of course on factors such as the intensity of the source lamp, but
tations yield reflected linearly polarised light (see Fig. 5.1). On arriving it also depends on the property known as reflectance. The reflectance of
at the surface of a polished section of an anisotropic ore mineral rotated a polished section of a mineral is defined as the percentage of incident
from extinction, the linearly polarised white light can be considered to light that is reflected from the surface of the section. This reflected light
separate into two coherent components (see Section 5.3.3). On leaving travels back up through the objective of the microscope and eventually
the surface the two components recombine and the ratio of their am- reaches the observer's eyes.
plitudes and their possible phase difference results generally in ellipti- The reflectance of a mineral is not simply a single number; it depends
cally polarised light. The light reflected from ore minerals appears as on variables such as the crystallographic orientation of the section
'white' light whose brightness and colour depend on the optical proper- through the mineral and the immersion medium between the specimen
ties of the mineral (Sections 5.1.1, 5.2). This 'white' light consists of a and the objective. Reflectance is related to two fundamental properties,
mixture of coherent rays of all wavelengths of visible light, but each namely the optical constants termed the refractive index and the absorp-
wavelength may differ in intensity and azimuth and nature of polarisa- tion coefficient. The relationship is expressed in the Fresnel equation:
tion. We can only tell that the reflected light is rather complex by
2
2
inserting and rotating the analyser and interpreting the resulting obser- R % = (n , - N ,) + k , x 100
0
vations (see Section 5.3). ' (n , + N ,)2 + k~ 1
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