Refractive indexes

Refractive indexes.

The refractive indices of minerals vary; this is an important hallmark, allowing the determination of minerals often with very similar properties.

Optically isotropic minerals, i.e.. amorphous, like opal, and crystallizing in a regular pattern, like a diamond, grenades, spinele i fluoryt, have only one refractive index, denoted by the symbol n. Optically anisotropic minerals belonging to other crystallographic systems have two or three values ​​of the main refractive indexes. In optically uniaxial minerals, these coefficients are given by the symbol na for the ordinary radius and nt for the extraordinary radius. Optically biaxial minerals have three refractive indices.

The value of double refraction is a characteristic feature, which in many cases facilitates the identification of minerals.

White light, e.g.. sunny, from an arc lamp or incandescent lamp (bulbs), it is not homogeneous. “It is made up of different wavelengths. After passing through the glass prism, the beam of white light rays is deviated from its original direction and split, giving a colorful spectrum. This phenomenon is called light dispersion. Violet waves are subject to the strongest deflection, for the weaker - blue, green, yellow, red light is the weakest refraction. The split light can be collected back into a white beam with a second prism.

One-color light, that is, monochrome, whose waves have a strictly defined length, has a characteristic color. Different colors of monochromatic light correspond to different wavelengths:

barwa wavelength in nm
Red 780—660
Orange 660—590
yellow 590—570
green 570—510
blue 510—450
Violet 450—380

The mentioned colors fall within the range of visible light. To the invisible light, to which the human eye does not respond, it should be infrared light (with a wavelength greater than red light) and ultraviolet light, that is ultraviolet (with a wavelength shorter than violet light).

The size of the refractive index depends on the wavelength, usually for violet light it is higher than for red light. Monochrome light is used for accurate refractive index measurements; the most common is yellow light (sodium), which is marked by placing the symbol of the sodium element Na below the letter n, denoting the refractive index: nNa. In addition, high-precision refractive index measurements also specify temperature and pressure, for its value also partially depends on them.

Gemstone light dispersion.

The speed difference of red and violet light passing through a substance is a measure of the dispersion of that substance. It is expressed by the difference of the refractive indexes of violet and red light. Diamond has a very high dispersion among precious stones; it is only surpassed by the dispersion of a few minerals, especially synthetic rutile. A dispersion similar to a diamond (0,044) has zircon (0,038). High diamond dispersion is an important factor causing the so-called. a fire so characteristic of this most precious gemstone. If you look at the diamond in a certain direction, you can see a yellow glow, a slight rotation of the stone can produce a red or blue glow. Bodies with little dispersion, like quartz or glass, they have no or very little fire.

Determining the values ​​of refractive indexes. After the passage of light from an optically rarer environment, i.e.. with a lower refractive index, to an optically denser environment, i.e.. with a higher refractive index, there is a refraction in the direction of perpendicular incidence of light. The angle of refraction is smaller than the angle of incidence. On the other hand, when light passes from an optically denser to a thinner environment, the angle of refraction is greater than the angle of incidence. In this case, when the light falls at an increasing angle, it will come out, that at a certain angle of incidence the refracted ray will run on the border of environments with different optical density. With even greater incidence angles, the light is completely internally reflected. There is then a strong glow, so important for gemstones.

The angle of incidence, under which the refracted ray no longer passes from an optically denser to an optically thinner environment, but it is completely reflected, is called the limit angle. The limit angle has different values ​​for different bodies, e.g.. for ordinary glass it is 48 °, while for diamond it is only 24 °. By measuring with instruments called refractometers the size of the boundary angle of the tested liquids or solids, we can determine their refractive indexes. In refractometers for less accurate, Serial determinations of the refractive index values ​​of the tested minerals are read directly from the scale.

If an empty test tube is immersed in a glass with water, it will shine, as if it were silver plated, which is the total external reflection of the light. The silvery reflection will disappear, when the test tube is full of water. The phenomenon of the appearance of a light streak in microscopic preparations at the border of two minerals with different refractive indexes is also the total internal reflection.. This is called. Beck's line or streak. To quickly find out about the value of a mineral's light factor, its crumb is placed under a microscope in a drop of resin, known as the Canadian Balm, with a known refractive index n = 1,54. When observing the grain of the mineral under the microscope, a bright light streak can be noticed at the interface between the mineral and the resin. When the microscope tube is lifted up, this streak moves towards the environment with a higher refractive index. The opposite phenomenon occurs when lowering the tube, because Becke's line shifts towards an environment with a lower refractive index. Low is the refractive index similar to the Canadian balsam, and tall - clearly taller than him, e.g.. 1,7.

The greater the refractive index difference, the more clearly Beck's Union appears. It is visible especially under high magnification and in not very bright lighting. Minerals with a very high refractive index, in contact with Canadian balsam or minerals with a balsam's refractive index, they are clearly visible and seem to be thicker than the neighboring minerals.

In the same way, a mineral is compared, to be marked with minerals identified on a different basis, e.g.. by color or type of cleavage. Tables are used for this purpose, in which the minerals are arranged according to an increasing refractive index.

The immersion method for the determination of refractive indices is directly applicable to the study of mineral grains with a size smaller than 0,04 mm. Liquid organic substances with known refractive indexes are usually used as immersion liquids.

Sometimes only one immersion fluid is used, with a high refractive index, which is diluted with an appropriate solvent with a lower refractive index, thus obtaining the possibility of testing the refractive indices of both liquids. In such a case, the refractive index of the liquid formed by mixing two liquids with known refractive indexes should be determined separately. Instead of measuring the refractive index of an immersion liquid with a refractometer, can be used indirectly, namely, to determine the density of the liquid. This value is strictly dependent on the ratio of the mixed liquids and on the refractive index, which can be read from the table. This method is especially applicable to aqueous inorganic solutions, with a large difference in density, like mercuric and potassium iodide solution (with a refractive index of 1.419-1.733, with a difference in density from 1,5 do 3,2) and a solution of barium and mercuric iodide (with a refractive index of 1.515-1.769).