Glass Fiber and Phytolith
A glass fiber and a phytolith are both present in this
field of view from a tapelift collected
in a school in eastern Washington State. The phytolith in the upper part of the
photograph is shaded on the left side
because its refractive index is below that of the mounting medium. The glass fiber
at the lower left is shaded on the the
right side because its refractive index is higher than the mounting medium. The
mounting medium in this case is the acrylic
adhesive of the tape with a refractive index of approximately 1.49.
Transmitted Oblique Off Crossed Polarized Light Illumination
Definition/Function:
The purpose of this image is to demonstrate the use of oblique illumination to quickly
distinguish between glass fiber and
silaceous phytoliths.
Significance in the Environment:
Characteristic Features:
Glass fiber and silaceous phytoliths are both isotropic, dark between crossed polarizing
filters. Oblique illumination
provides a very quick way to distinguish the difference between most glass fibers and
silica phytoliths. The glass fiber
genrally has a refractive index greater than the tape adhesive. The tape adhesive
(acrylic) has a refractive index of about
1.49. Most common glass fiber has a refractive index of 1.51 or higher, often much
higher, fused quartz fiber is the only
common exception with a refractive index of 1.48. Oblique illumination results in a
shadow effect and the shadow falls
toward the source of the light or away from the source of the light depending on whether
the refractive index of the
particle is higher or lower than the mounting medium. Phytoliths, with a refractive
index of 1.48, have a refractive index
below the acrylic adhesive. In this photograph the phytolith in the upper part of the
photograph is shaded on the left side.
The glass fiber at the lower left is shaded on the the right side. This shows that the
glass fiber has a higher refractive
index than the phytolith. The intensity of the shadow can be used in the same way as
"relief" with normal brightfield
illumination but with the advantage that no change in focus in necessary to characterize
the Becke Line. This is a great
aid in quickly characterizing particles as one scans the sample. On closer inspection of
the phytolith it can be seen to
have a rough or irregular surface. That is not consistent with glass fiber whose
formation is a result of surface tension.
Associated Particles:
References:
