Extraordinary Ray
Definition and meaning of Extraordinary Ray in chemistry.
Extraordinary ray refers to one of the two light rays produced when a beam passes through an optically anisotropic (birefringent) crystal, distinguished from the ordinary ray by having a refractive index that changes with propagation direction relative to the crystal's optic axis.
In more detail
In a uniaxial birefringent crystal, unpolarized light splits into two linearly polarized components: the ordinary ray (o-ray), polarized perpendicular to the optic axis with a constant refractive index n_o that obeys standard Snell's law, and the extraordinary ray (e-ray), polarized in the plane containing the optic axis, whose refractive index n_e(θ) varies smoothly between n_o and the principal value n_e depending on the angle θ between the propagation direction and the optic axis. Because n_e changes with direction, the extraordinary ray's wavevector and its energy-flow (Poynting vector) direction generally diverge, an effect called walk-off. This double refraction is the physical basis of polarizing optics such as the Nicol prism and is used to characterize anisotropic crystalline and mineral samples.
Key facts
| Field | Physical Chemistry |
|---|---|
| Paired phenomenon | Ordinary ray (o-ray) |
| Refractive index | n_e(θ), direction-dependent |
| Classic example material | Calcite, CaCO3 |
Viewing printed text through a clear calcite (CaCO3) crystal shows two overlapping images, one formed by the ordinary ray and one, displaced and shifting position as the crystal is rotated, formed by the extraordinary ray.
Frequently asked questions
How is the extraordinary ray different from the ordinary ray?
The ordinary ray has a single refractive index n_o in all directions and obeys ordinary Snell's law, while the extraordinary ray's effective refractive index depends on the angle between the light's propagation direction and the crystal's optic axis, so it can bend at angles Snell's law with a fixed index would not predict.
Where does the extraordinary ray occur?
It occurs in optically anisotropic (birefringent) crystals, such as calcite, quartz, and ice, whenever light travels through the crystal in a direction other than exactly along the optic axis.