Question

Explain two methods to determine the refractive index (sketch) and
specify the achievable accuracy.

Answer 1

'There are several methods of determining the refractive indices of minerals as follows

Nicol prism:

This is a device to produce and analyze polarized light through the principle of double refraction. It consists of a rhombohedron of calcite whose length is three times the breadth. The top and bottom surfaces are ground down to give an angle of 68° with the long edge. The block is then cut along the smaller diagonal and cemented together again with the help of a layer of Canada Balsam whose refractive index is 1.537, which acts as a rarer medium for an ordinary ray (R.I. 1.658) and a denser medium for the extraordinary ray (I.516).

Since the ordinary ray travels from denser to rarer medium, it is totally internally reflected, as it is so arranged that the angle of incidence at the Canada Balsam layer is greater than the critical angle for the Ordinary ray. But in case of extraordinary ray, since it travels from rarer to denser medium it passes through the nicol with little deviation and vibrates parallel to the short-diagonal of the crystal.

These are called nicol prisms which are used in the petrological microscopes both for producing and analyzing the polarised light and thus aids in the identification of minerals.

Pleochroism:

It is an important optical properties of minerals in thin- section, in which the change in quality and quantity of colour is observed on rotation of the stage through an angle of 90°.

The colour shown by a mineral in this section results from the absorption of certain 'colours' (wavelengths) from the incident white light; the resulting transmitted light being complementary in colour to that absorbed. Thus pleochroism is defined as the varia­tion in colour resulting from differential absorption of wavelength in different directions.

Becke-line method:

The principle involved is the total reflection of light incident at more than the critical angle when passing from a mineral of greater index to that of a lesser index in thin section. Accordingly a portion of the beam is deflected towards the mineral with greater index which results in a thin band of light visible just inside the boundary of greater index. This band of light is called the Becke-line, which moves towards the mineral of greater refractive index when the microscope-tube is slightly raised and the reverse effect is produced when the tube is lowered.

Oblique-illumination method:

Oblique illumination is made by cutting off half of light by using a finger or card below the stage. Thus half of the field remains illuminated and one side of the mineral will be dark and the; opposite side lighted. It has been observed that of the rays pass from a mineral of higher index into a mount of lower index, they are concentrated by refraction and form a light band; if they pass from the mount of lower index into the higher mineral they are spread out by refraction and so produce a shadow. In general "if the shadow appears on the side away from the dark half, the refractive index of the mineral in question is greater than that of the adjoining medium and vice versa"