Thank you for making a response to my question and for referring me to texts explaining birefringence.
O-ray and e-ray are much discussed topics in gemological education, but I have my doubts if an average student would fully understand their characters.
A calcite rhomb is always used to demonstrate that o-ray is stationary and that e-ray is the moving image around it. This exercise serves well to show the phenomena but the answer has not been addressed.
When light falls on a piece of anisotropic material, a plane of light incidence will be formed with the optic axis of that material and the light is apt to split into two components vibrating perpendicular to each other at unequal speeds, one of which is the o-ray perpendicular to the incident plane while the other one is the e-ray lying parallel to this plane. As the angle of light incidence changes, the size of the incident plane will change too with a result that e-ray value will vary. When light comes into the material parallel to its optic axis, the incident angle is zero, the incident plane disappears and therefore no more e-ray at that point. On the contrary, o-ray is not attached to the incident plane, it's value is unchanged(not affected )regardless direction of incident ray.
This phenomena can be further exemplified : -
1) when the sun shines right above you at 90°, you cannot see your shadow. But as the sun shines upon you at an angle, you will see your shadow that prolongs according to the incident angle. The shadow is the e-ray that changes on incident direction. The shadow of your body width will stay unchanged as is the case of o-ray.
2) when a pencil is put into a glass filled with water, it appears bent while at an angle off the normal. You see double refraction. But when the pencil stands at right angle into this glass of water, or when it is floated on the water, you will not see it bent. It means at these two points, there is only one refractive index.
The above examples will also serve to explain straight extinction, oblique extinction and the lattice parameters of the six crystal systems.