REFRACTION

As light passes from one substance or medium to another it is refracted, that is, the transmission direction is changed. The velocity of light is a funciton of the medium through which is passes, being slower in a more dense mediium. Since the frequency of vibration of light remains constatn, the wavelenght must change as light enters a different medium to accommodate the change in velocity. The change in wavelength upon entering a different medium gives rise to refraction, or the bending of light. The refractive index of a mineral is determined by th erelation of the velocity of light in air as compared to the velocity of light in th emineral:

BIREFINGENCE

The difference in refractive index of the two rays (refractive index of slow ray minus refractive index of fast ray) is defined as birefringence. By definition, birefringence is a numeric value, not a visible property. Interference color is the visual manifestation of birefringence using polychromatic light. Although the interference color may change, the birefringence of a mineral remains constant.
Birefringence is dependent on several factors:

• Mineral species (quartez=0.009 calcite=0.2).
• Wavelength of incident light (minimal effect).

INTERFERENCE COLOR

Interference color results from the unequal transmission by the analyzer of the components of white (polychromatic) light. Interference colors are produced only in anisotropic minerals under cross-polarized light. When the grain is not at extinciton, the mineral will divide the plane-polarized light into rays vibrating at right angles to each other. These two rays, whe resolved along the vibration direction of the analyzer will interfere. The interference is due to the fact that the two rays travel trough the mineral with different velocities and wavelengths. When they emerge from the mineral the two waves recombine with their original velocity and wavelength. But in the process there is a phase difference in the waves. If the phase difference equals n wavelength (n=1,2,3--), the two rays when resolved will interfere (cancel) and no light is transmitted through the analyzer. Since white light is polychromatic (many wavelengths), only certain wavelengths will be eliminated in this way. At the same time, wavelengths whose phase difference equals 1/2 n will be reinforced, resulting in maximum brightness for that color. The combination of elimination of some wavelengths and the accentuation of others results in the interference colors observed in minerals under cross-polarized light. This is an optical phenomena and not an inherent characteristic of the mineral.

Interference color is a function of:

• Mineral thickness (distance light waves travel through the crystal)
• Birefringence (difference between the two indices of refraction)
• Crystal orientation with reference to the plane of polarization

This page (http://www.ces.ncsu.edu/plymouth/programs/behavior.html) created by
Vera MacConnell, Research Technician, I on November 11, 1997.
Last Updated on November 13, 1997.