The ordinary ray (o-ray) is the ray in a uniaxial stone that vibrates across the optic axis. It has the same refractive index, ω or nₒ, in every direction.
In more detail
The ordinary index ω is 2.648 in moissanite, 1.770 in ruby and sapphire, 1.583 in emerald and 1.544 in amethyst. With the table cut across the axis, the o-ray gives one of the two refractometer edges.
How we measure it
Our Dichroscope shows the o-ray color in one window when the dichroscope lines up with the stone. In ruby the o-ray is purplish red; in emerald it is yellowish green.
Related terms
- Extraordinary ray: In a uniaxial stone, the extraordinary ray (e-ray) vibrates in the plane of the optic axis and the light path. Its refractive index changes with direction, from ω along the axis to ε across it.
- Birefringence: Birefringence is the difference between the two refractive indices of a doubly refractive stone. It shows how strongly the stone splits light into two rays.
- Optic axis: An optic axis is a direction in a doubly refractive crystal along which light does not split into two rays. Seen along it, the stone behaves as if it were singly refractive.
- Pleochroism: Pleochroism is the property of a colored doubly refractive stone to show different colors in different directions. It happens because the stone absorbs its rays differently.