The Photoelastic Investigation of Three-Dimensional Stress and Strain Conditions Page: 3 of 19
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N.A.C.A. Technical Memorandum No. 824
But such substances may lose this property on becom-
ing deformed. The propagation of light in a small parti-
cle of the substance then resembles the propagation of
light in a crystal.
For example: Glass which is optically isotropic be-
fore deformation, manifests under unidirectional stress
the optical property of a uniaxial crystal (reference 2).
Every point of the ether in the substance acted upon by
the light becomes the simultaneous source of two light
waves - one having the shape of a sphere, the other that
of the surface of an ellipsoid of revolution. For any di-
rection of rays through the point of origin of these light
waves there are with one exception, two rays of unequal
velocity. The light of each ray vibrates in only one plane
and perpendicular to the direction of the rays. The planes
of vibration of the two rays of one direction are at right
angles to each other. One single ray velocity exists only
in the direction through the point of origin of the light
waves and the two points of contact of the wave surfaces.
This particular direction is called the optic axis.
In general the propagation of light in a small arbi-
trarily deformed particle of transparent matter resembles
the propagation of light in an optically biaxial crystal
Every point of the ether in the substance which is
acted upon by the light becomes the source of light waves
with a two-sheeted wave surface of the fourth order.
By the use of a different presentation this surface
can be replaced by a more simple one, the so-called
Fresnel ellipsoid (fig. 1).
The two ray velocities belonging to each ray direc-
tion S and their corresponding planes of vibration are
obtained by passing a plane E perpendicular to S
through the center P of the ellipsoid. The half-lengths
of the principal axes L1 and L2 of the elliptical sec-
tion give the two ray velocities vi and v2. The direc-
tion of the principal axis L1 (L2) of this elliptical
section together with direction S define the plane of
vibration of the light ray v1 (v).
The half-lengths of the principal axes of Fresnelts
ellipsoid correspond to the so-called three principal
light velocities v1, v1i, and viiI. Two planes can be
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Oppel, G. The Photoelastic Investigation of Three-Dimensional Stress and Strain Conditions, report, April 1937; (https://digital.library.unt.edu/ark:/67531/metadc63373/m1/3/: accessed April 18, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.