Optical Property of Gem Stones
In optical properties of gemstones we study the propagation and behaviour of light. The Light is that energy by which we feel sensation of vision.
Optical properties of a gem plays an important roll in its beauty and value. Light produce different colour sensation when it vibrate with different frequencies. Light is emitted from source in straight lines. Now when light strikes an object it is either absorbed, reflected or refracted. The reflected light from rough source is scattered in all directions. Now these qualities of absorption, reflection, refraction or polarization is very important for the optical study of gemstones.
In gemstones following are the points comes under the study of optical properties
- Total internal reflection
- Critical angle
- Refractive index
- Polarised light
- Double refraction
- Optic axis (Uniaxial-Biaxial)
- Colour & other effects
Total Internal Reflection
When a ray of light enters from denser to rarer medium it bents away from the normal. While this it makes an angle called angle of incidence. As this angle of incidence is increases the angle of refraction also increases. when the light ray strikes at an angle greater than the angle of incidence the light remain in the same denser medium. This turning of rays back into the stone is called total internal reflection. This is the cause of sparking in the gemstone.
There is a particular angle of incidence, for which corresponding refracted ray is at 90? to the normal. This is known as Critical angle
It is the ratio of velocity of light in air to the velocity of light in that medium. The quantity representing the refractive power in air is equal to 1 taken as standard.
Measurement of refractive index: Refractometer is an instrument which measure the critical angle which depends upon the index of refraction.
It has a lead glass prism with high refractive index 1.86 with low hardness of this glass prism i.e.5 is the most important part of this reflactometer
Wave theory gave us that a ray of light vibrates in all directions at right angles to direction of travel & if this light is passed through a polarised sheet , the out coming light will vibrate only in one direction perpendicular to the source of propagation which is called polarised light.
Crystals belonging to other than isometric system shows double refraction when an incident ray strike them at an angle other than normal it splits in to two rays each with different velocity and different index of refraction and each taking a different path in the stone. Substances with this type of split are called DR or doubly refractive or anisotropic
Optic Axis (Uniaxial-Biaxial)
It is a single refractive direction in a doubly refractive stone. If a material is having one such axis, it is called uniaxial. If a material is having two such axis, it is called biaxial. Stones belonging to tetragonal, hexagonal crystal structure have one such axis, therefore they are uniaxial. Material belonging to orthorhombic, monoclinic, triclinic crystal systems has two such optic axes and is biaxial.
Colour & Other Effects
Gemstones shows various colours due to their optical properties. These are as:
The selective absorption of polarized light by coloured crystals is called Pleochroism, rather literal meaning is “pleo” means several and “chroism” means colour. Stones which are transparent, coloured & have double refraction shows this property. When light ray enters in a gemstone having double refraction, it split into either two or three rays. Each ray absorbs different colour of different shades of the same colour, and as we rotates the stone, we can see the different colours shades. This property called as Pleochroism. It is occurred in coloured stone with doubly refractive crystals & not in colourless crystals of any description.
In dichroism only two colours are noted, as in tetragonal and hexagonal crystals, which contain only two polarized light components, this is known as dichroism.
In orthorhombic, monoclinic & triclinic the crystal structure is complex which produce pairs of polarized light components that differ in character. In these crystals three distinct hues are observed and it is known as trichroism.
Many gems exhibit the property of fluorescence i.e. absorbing light of one wavelength and emitting lights of another wavelength or colour when they are exposed to ultra-violet light.