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electromagnetic radiation energy, having wavelengths of between 400nm and 700nm, which is detected by human eyes
because we normally use our eyes to detect light radiation, light provides the most convenient means for various methods of testing and identifying most gem materials (optical testing) |
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| the direction taken by a single wave of light |
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| the direction in which teh wave of light vibrates as it travels |
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| the distance between the top of one wave and the top of the next |
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| the number of waves passing a point in a certain time |
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the range of electromagnetic radiation wavelengths from the longest, low frequency radio waves, through diminishing wavelengths to infrared, visible light, ultraviolet, x-rays and gamma rays to the shortest of all, very high-frequency cosmic rays
in gemmology, the word spectrum is often used to denote merely the visible light spectrum and the absorption spectrum |
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| Electromagnetic Radiation |
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| transmitted electromagnetic energy, of any part of the electromagnetic spectrum, spoken of as waves or rays |
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those physical properties relating to behaviour under visible light
Ex) transparency, reflection, refraction, color, luster |
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| the return by an internal or external surface of some of the light which falls on that surface |
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Incident
(light, ray, radiation) |
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Definition
electromagnetic energy that meets a surface is said to be 'incident'
incident light is either reflected, absorbed or refracted at the surface |
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| the angle between the incident ray and the normal to the point of incidence at the surface |
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a term used when describing paths of light rays during reflection and refraction
the 'normal' to a surface is an imaginary line perpendicular to a surface at the point of incidence |
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| the change in the direction in which light travels when it passes from one medium into another of differing optical density (except where incident at 90° to the interface) |
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| the angle between the refracted ray and teh normal to the point of incidence at the surface |
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| a ratio which indicates the 'optical density' of a substance; it is related to the angles of incidenc and refraction of lgiht by Snell's Law of Refraction and can be determined by measuring the critical angle |
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| a complex property which manifests itself in the slowing down of light on its entry into a mdium denser than the one in which it was previously travelling - the greater the optical density, the greater the reduction in velocity |
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| at the interface between two media of different optical density, when teh angle of incidence of a ray of light from within the optically denser medium is increased to such an extent that a refracted ray of light passes along the interface, then this angle of incidence is called the critical angle |
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| an optically isotropic material exhibits the same optical properties in all directions through the material |
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| the optical doubling effect caused by the splitting of visible light into two separate componenets by an optically anisotropic material |
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an optically anisotropic material exhibits different optical properties in different directions through the material
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| when there is no order to the vibration directions coming from a light ray |
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| Plane Polarized Light (PPL) |
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| light whose waves all vibrate parallel to a single plane |
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| the maximum numerical difference between the two refractive indices of an optically anisotropic material |
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| an optic axis is a direction, through an anisotropic crystal structure, parallel to which single refraction occurs |
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an anisotropic crystal having one direction (one optic axis) parallel to which there is single refraction
materials belonging to the trigonal, tetragonal and hexagonal crystal systems |
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an anisotropic crystal having two directions (optic axes) along which there is single refraction
materials belonging to the orthorhombic, monoclinic and triclinic crystal systems |
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Term
| Total Internal Reflection (TIR) |
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Definition
occurs when incident light meets the interface between two media of differing optical density at an angle greater than the critical angle
the light remains within the optically denser medium and obeys the laws of reflection
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"cat's-eye effect"
an internal reflection effect which appears as a bright band of light across the surface of certain gemstones when cut as cabochons
the reflection is from fine inclusions, all parallel to one direction, and is visible when the based of the cabochon is cut in parallel with the direction of the inclusions |
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Term
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Definition
a star-like reflection of intersecting bands of light which is seen from the surface of certain gems when they cut in a particular crystallographic orientation as cabochons
it is an internal reflection effect caused by reflections from two or more sets of parallel fibrous or channel inclusions |
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the degree of brightness resulting from reflection of light by a gemstone when viewed through the crown facets
it is made up of light reflected from the pavillion facets and from the surface of the crown |
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| a popular term for the spectral colors displayed by a gemstone with a significant degree of dispersion |
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| an optical instrument used in testing transparent gemstones to distinguish between isotropic and anisotropic materials and to detect strain in materials |
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a general term which includes modification of light propagation
Ex) plane polarized light vibrates parallel to a single plane |
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| a substance which causes polarization of light |
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| two polarizing filters are 'crossed' when arranged one in front of the other and rotated until a minimum of light is transmitted to teh eye |
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Definition
| the lack of visible light transmission through a transparent anisotropic material at certain position when observed between crossed polarizers |
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| Anomalous Extinction Effect |
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Definition
when viewing certain gemstones between cross polarizers, a pattern of light and dark extinction zones, band or cross shapes may be seen within some stones; these may move across the stone on rotation
theis effect is due to internal strain in the material which causes a certain amount of anisotropy in otherwise isotropic structure |
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| an instrument which works on the principle of total internal reflection and which is used to measure the refractive index of gemstones |
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light of one color composed of a single wavelength or a very narrow band of wavelengths
in gemology, monochromatic yellow light ('sodium light') is used for accurately determining refractive index |
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Definition
| a direction, through an anisotropic material, parallle to which single refraction occurs |
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| this ray obeys the ordinary laws of refraciton; the RI of the o-ray is constant regardless of tis direction of travel in the crystal |
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this ray does not obey the ordinary laws of refraction; the RI of the e-ray varies
vibrates parallel to any plane containing the optic axis |
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| the upper polarizing filter of the polariscope |
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| the lower polarizing filter of the polariscope |
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| patterns of light and dark which reveal the directions of the optic axes and whether a gem is uniaxial or biaxial |
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