What is "optic axis of a crystal"

Axis \Ax"is\, n.; pl. Axes. [L. axis axis, axle. See Axle.] A straight line, real or imaginary, passing through a body, on which it revolves, or may be supposed to revolve; a line passing through a body or system around which the parts are symmetrically arranged. 2. (Math.) A straight line with respect to which the different parts of a magnitude are symmetrically arranged; as, the axis of a cylinder, i. e., the axis of a cone, that is, the straight line joining the vertex and the center of the base; the axis of a circle, any straight line passing through the center. 3. (Bot.) The stem; the central part, or longitudinal support, on which organs or parts are arranged; the central line of any body. --Gray. 4. (Anat.)

1. The second vertebra of the neck, or vertebra dentata.

2. Also used of the body only of the vertebra, which is prolonged anteriorly within the foramen of the first vertebra or atlas, so as to form the odontoid process or peg which serves as a pivot for the atlas and head to turn upon.

   5. (Crystallog.) One of several imaginary lines, assumed in describing the position of the planes by which a crystal is bounded.

   6. (Fine Arts) The primary or secondary central line of any design.

   Anticlinal axis (Geol.), a line or ridge from which the strata slope downward on the two opposite sides.

   Synclinal axis, a line from which the strata slope upward in opposite directions, so as to form a valley.

   Axis cylinder (Anat.), the neuraxis or essential, central substance of a nerve fiber; -- called also axis band, axial fiber, and cylinder axis.

   Axis in peritrochio, the wheel and axle, one of the mechanical powers.

   Axis of a curve (Geom.), a straight line which bisects a system of parallel chords of a curve; called a principal axis, when cutting them at right angles, in which case it divides the curve into two symmetrical portions, as in the parabola, which has one such axis, the ellipse, which has two, or the circle, which has an infinite number. The two axes of the ellipse are the major axis and the minor axis, and the two axes of the hyperbola are the transverse axis and the conjugate axis.

   Axis of a lens, the straight line passing through its center and perpendicular to its surfaces.

   Axis of a microscope or Axis of a telescope, the straight line with which coincide the axes of the several lenses which compose it.

   Axes of co["o]rdinates in a plane, two straight lines intersecting each other, to which points are referred for the purpose of determining their relative position: they are either rectangular or oblique.

   Axes of co["o]rdinates in space, the three straight lines in which the co["o]rdinate planes intersect each other.

   Axis of a balance, that line about which it turns.

   Axis of oscillation, of a pendulum, a right line passing through the center about which it vibrates, and perpendicular to the plane of vibration.

   Axis of polarization, the central line around which the prismatic rings or curves are arranged.
   --Brewster.

   Axis of revolution (Descriptive Geom.), a straight line about which some line or plane is revolved, so that the several points of the line or plane shall describe circles with their centers in the fixed line, and their planes perpendicular to it, the line describing a surface of revolution, and the plane a solid of revolution.

   Axis of symmetry (Geom.), any line in a plane figure which divides the figure into two such parts that one part, when folded over along the axis, shall coincide with the other part.

   Axis of the equator, ecliptic, horizon (or other circle considered with reference to the sphere on which it lies), the diameter of the sphere which is perpendicular to the plane of the circle.
   --Hutton.

   Axis of the Ionic capital (Arch.), a line passing perpendicularly through the middle of the eye of the volute.

   Neutral axis (Mech.), the line of demarcation between the horizontal elastic forces of tension and compression, exerted by the fibers in any cross section of a girder.

   Optic axis of a crystal, the direction in which a ray of transmitted light suffers no double refraction. All crystals, not of the isometric system, are either uniaxial or biaxial.

   Optic axis, Visual axis (Opt.), the straight line passing through the center of the pupil, and perpendicular to the surface of the eye.

   Radical axis of two circles (Geom.), the straight line perpendicular to the line joining their centers and such that the tangents from any point of it to the two circles shall be equal to each other.

   Spiral axis (Arch.), the axis of a twisted column drawn spirally in order to trace the circumvolutions without.

   Axis of abscissas and Axis of ordinates. See Abscissa.

The optic axis of a crystal is the direction in which a ray of transmitted light suffers no birefringence (double refraction). Due to the internal structure of the crystal (the specific structure of the crystal lattice, the form of atoms or molecules of its components), light behaves differently when propagating along the optic axis than in other directions. Light propagating along the optic axis of a uniaxial crystal (e.g. calcite, quartz), has no unusual results. Light propagates along that axis with a speed independent of its polarization. If the light beam is not parallel to the optic axis, then the beam is split into two rays (the ordinary and extraordinary) when passing through the crystal. These rays will be mutually orthogonally polarized.

The optic axis of a crystal is a direction rather than a single line. If a ray in this direction suffers no birefringence, neither will all parallel rays. A crystal with only one optic axis is called a uniaxial crystal. Crystals are classified according to the number of optic axes (uniaxial, biaxial) they have. A uniaxial crystal is isotropic within the plane orthogonal to the optic axis of the crystal.

The refractive index of the ordinary ray is constant for any direction in the crystal. The refractive index of the extraordinary ray varies depending on its direction. The ordinary passes undeviating through the crystal and show normal optical properties ( reflection and refraction), but the extra-ordinary ray passes deviating through the crystal. Non-crystalline materials have no double refraction and thus, no optic axis. Some solid materials under specific conditions can demonstrate double refractions and optic axes.