Backe Line Test

The relief indicates the difference between the RI (Refractive Indices) of the mineral grains and the surrounded cement material. The RI of the grain can be Greater than, Less than or equal to that of the surrounding material. “Backe Line Test” is carried out to determine weather the RI of mineral is higher or lower than the surroundings or to determine the RI of a material.

The Becke Line Method (Comparative measurement of RI)

In the Backe Line Test a high power objective is used and the iris diaphragm of the microscope below the stage is partly closed. A tapering edge of a mineral grain is selected and brought into focus. If now microscope tube is raised, a narrow line of light called “Backe Line” will appear at the grain boundary.

• These backe line moves towards the medium of higher refractive index when the tube is raised.

“This method is very much useful in case of isotropic minerals as they have only one RI”

Accurate measurement of RI

In order to determine the accurate value of refractive index of a mineral, a set of suitable liquids of known refractive index is required. These liquids are called as “Immersion media” .

1. A mineral grain is immersed in a drop of liquid of known refractive index.
2. By the use of backe line, the RI of mineral and liquid is compared.
3. If the Backe line moved into the mineral grain, a new mount is made by using a liquid of higher RI.
4. This procedure is repeated untill the exact match between the liquid and mineral grain is obtained.
5. In such a case the RI of mineral will be equal to that of the liquid.

Special notes :

• If the focal distance is decreased, rather than increased, then the opposite results will be obtained, that is with decreasing focal distance the bright Becke line will move into the substance with lower refractive index.
• The refractive indices for grains (and also oils) are reported for a specific wavelength of light. That wavelength is 589 nm, which corresponds to yellow.
• Since we are using white light as an illuminator for our grain, the Becke line will be different for different wavelengths or colors of light.