In fact, the only etchant known to be even marginally successful is hydrofluoric acid ( HF ), which etches slowly and predominantly in the direction of the crystal optic axis.
42.
However, if the viewing angle is slightly away from the optic axis, the centre of the cross will revolve / orbit around the central point as the stage is rotated.
43.
In a uniaxial crystal, different polarization components of a beam will travel at different phase velocities, " except " for rays in the direction of what we call the optic axis.
44.
Thus the optic axis has the particular property that rays in that direction do " not " exhibit birefringence, with all polarizations in such a beam experiencing the same index of refraction.
45.
The terms " ordinary " and " extraordinary " are still applied to the polarization components perpendicular to and not perpendicular to the optic axis respectively, even in cases where no double refraction is involved.
46.
In these devices, the source and detectors of the light are not located on the optic axis of the mirror, so the use of a true paraboloid of revolution would cause a distorted image.
47.
An optic axis is a direction in which light travels through a crystal such that the speed is the same for all directions of polarization for light of any given wavelength ( i . e . color ).
48.
Two of the inter-axial angles are 90 degrees, while the third angle, between the optic axis and the plane containing the other two axes, is unequal to the other inter-axial angles.
49.
In the analysis of these systems, it is common to consider tangential rays ( as defined above ), and rays in a meridional plane ( a plane containing the optic axis ) perpendicular to the tangential plane.
50.
No optical system is perfectly shift invariant : as the ideal, mathematical point of light is scanned away from the optic axis, aberrations will eventually degrade the impulse response ( known as a coma in focused imaging systems ).
