Optical Properties Flashcards
What is Anomalous Double Refraction (ADR)?
Anomalous Double Refraction (ADR) is where a gemstone that is Singly Refractive appears to exhibit double refraction, especially when viewed through the polariscope.
This is due to defects or strain within the crystal lattice structure, that can make it appear ‘slightly’ doubly refractive.
A common example is some garnets, paste, or flame-fusion synthetic spinel.
What is Single Refraction (SR)
Single Refraction or Isotropism is where the ray of light that enters the stone is slowed down and refracted as a single ray, therefore entering and leaving the gemstone as 1.
The Ray follows Snell’s Sine Law of Refraction.
SR gemstones are amorphous substances (e.g. paste and natural glasses) and crystals in the cubic system.
What is Double Refraction (DR)
Double Refraction or anisotropism, is where the ray of light is slowed down and refracted as two different rays (the ordinary and the extraordinary).
The rays will be travelling at different speeds and different directions.
The rays will be plane polarised at right angles to each other.
All crystal systems other than cubic have double refraction.
What is plane polarisation?
Ordinary light vibrates at all planes. Plane polarisation of light is when vibrations of light take place in one plane only, at right angles to the wavefront.
There are three main methods to polarising light.
1. Transmission through a doubly refractive (DR) material
2. Absorption
3. Reflection
