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.
In more detail
The extraordinary index ε is 2.691 in moissanite, 1.762 in ruby and sapphire, 1.577 in emerald and 1.553 in amethyst. Along the optic axis, the e-ray has the same index as the o-ray, so there is no split.
How we measure it
The simulator traces the e-ray with 1/n(θ)² = cos²θ/nₒ² + sin²θ/nₑ². Our Dichroscope shows its color: orangy red in ruby, bluish green in emerald.
Related terms
- Ordinary ray: 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.
- 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.