In a wave, what are: the direction of propagation; the direction of vibration; the wavelength; the wavehight
The direction of propagation is the direction in which the wave travels;
The direction of vibration is the direction that a wave vibrates, always 90° to the direction of propagation;
The wavelength is the distance between two equivalent points of a wave (ie two peaks);
The wavehight is the distance between the peak and lowest point of a wave
In visible light, what determines brightness and what determines colour?
Brightness is determined by wave hight;
Colour is determined by wavelength;
What wavelength does visible light occupies on the electromagnetic spectrum?
Visible light ranges from 400 nanometres (violet) to 700 nm (red)
What is reflection?
Reflection is the return of all, or some of, the light that falls on a surface
Given an incidence angle of [I], how much is the angle of reflection [R]?
The law of reflection states that the angle of reflection is equal to the angle of incidence, so [R] = [I]
What is optical density?
Optical density is the extent to which a trasparent material alters the speed of light travelling through it
What is refraction?
Refraction is the bending of light when it passes from one material to another; this bending is due to the change in light speed
When light passes from a optically denser material to a less dense one (ie going from a gem to air), what happens to the refraction angle?
When going from a denser material to a less dense one, light bends away from the normal, so the refraction angle is larger than the incidence angle
What does optically isotropic means? What materials can be considered optically isotropic?
Optically ortotropic means that the optical properties of the material are the same in every direction
Amorphous materials and crystals in cubic system are optically isotropic
What is double diffraction? What type of materials show double diffraction?
Double diffraction is the splitting of the light entering a material; these rays travel with different speeds and slightly different directions
Optically anisotropic materials, such as non-cubic crystal systems, show double diffraction
What is the refractive index? What instrument is used to calculate it?
The refractive index is the ratio between the speed of light in air and the speed of light in a material
The refractive index is measured with a refractometer
What is total internal refraction? What is the critical angle?
Total internal refraction occurs when the refraction angle is 90° from the normal
The critical angle is the minimum incidence angle needed to achieve total internal refraction
What is the difference between polarized and non-polarized light?
In non-polarized light, light vibrates in every possible planeperpendicularly to its propagation direction; in polarized light, light vibrates on a single plane perpendicular to the propagation direction
What happens to non-polarized light entering an anisotropic gem?
The light is split in, usually, two rays which are polarized perpendicularly to each other
What is the optical axis in anisotropic gems?
The optical axis is direction in an anisotropic gem along which the light does not divide into polarized rays
Regarding optical axis, which crystal systems are uniaxial?
Which crystal systems are biaxial?
Tetragonal, hexagonal and trigonal systems are uniaxial
Orthorombic, monoclinic and triclinic are biaxial
What is the cat eye effect?
The cat eye effect is a multiple internal reflection that appears as a single bright line that moves accross the curved surface of a gem
What can cause the cat eye effect?
The cat eye effect is caused by:
-Long and thin inclusions;
-Inclusions arranged parallel to each other;
-The surface of the gem is cut in a way that shows this effect, such as sphere or cabochon
What gems can display cat eye?
Almost all gems, except diamonds, can display cat eye effect; in particular this effect happens more often with chrysoberyl, quartz and tourmaline
What is asterism?
Similar to cat eye effect, asterism is a multiple internal reflection that appears as a two or more bright lines, due to multiple sets of parallel inclusions
Where does asterism shows more often?
Asterism is frequent in corundum, but can also be seen in spinel, almandine garnet and rose quartz
What is aventurescene?
Aventurescene is a sparkling appearance produced by inclusions
What is brilliance? What does it depend on?
Brilliance is the total amount of light return of light that is reflected from the crown and pavillion facets
Brilliance is affected by transparency, claricy, lustre and polish
Why for some colored gems brilliance is not the main aspect to optimize?
Colored gems require larger pavillion angles to allow for total internal reflection and might make the stone too deep
