Refraction of light in the crystal.
Light rays passing through a vessel filled with water are refracted, if they do not hit the water surface at right angles. The greater the angle of incidence of the light rays, the greater their deviation from the original direction. If you put a straight rod in the water, an illusion arises, as if it had broken at the point where the water surface came into contact with air. This is also the misjudgment of depth in the clear water of a stream or lake; always the bottom seems closer than it really is.
A similar phenomenon can be observed in the glass plate, through which a beam of light is passed: it breaks down, assuming the original direction after leaving the plate. Light behaves in the same way in a transparent crystal of some substance crystallizing in a regular system, e.g.. in rock salt or fluorite crystal, which are optically isotropic bodies.
In a calcite plate, being an optically anisotropic body, i.e.. not crystallizing in a regular system, but in another, a different phenomenon can be observed. The beam of light rays in calcite is not only refracted, but also splitting into two bundles, which, when leaving the calcite plate, run parallel to the original direction. This phenomenon also arises then, when a beam of light rays falls perpendicularly on the surface of the crystal. One of the rays then passes without changing its direction, the other, on the other hand, is deviating. After leaving the plate, both rays run parallel to the original direction.
Refraction of light in calcite: a - oblique incidence of light rays, b - perpendicular incidence of light rays, c - with different thickness of the crystal, d - when rays pass through two crystals.
If the transparent calcite crystal is put on the paper, on which there is a black point, two points are visible. The greater the thickness of the calcite, the greater will be the distance between the two points (this is explained in the figure). From the thickness of the optically anisotropic body, direction depends on the distance between the rays, which, leaving this body, continue parallel to the original direction of incidence of light on its surface. The print placed under the calcite crystal is visible as a double print. This property of double refraction in optically anisotropic crystals is called birefringence. Rays, which are formed in an optically anisotropic plate do not have the same properties. The ray that obeys the ordinary laws of refraction is called the ordinary ray, the other, the extraordinary ray. The ordinary ray behaves like this, as in an optically isotropic environment and has a constant refractive index, while the refractive index of the extraordinary ray has a magnitude depending on the direction. For example, the refractive index of calcite for an ordinary ray is 1,65, and for the extraordinary radius 1.48-1.65. The body, in which the differences in refractive index are large, e.g.. kalcyt (1,65—1,48 = 0,17), is called highly light-refracting, that is, strongly birefringent; bodies with a small difference in refractive index, e.g.. quartz (1,544—1,553 = 0,009) or orthoclase (1,526—1,518 = 0,008), are called weakly birefringent.
Light, that has passed through the calcite plate has properties different from ordinary light. Light with these different properties is called polarized light. Polarized light vibrates in one plane, in contrast to ordinary light, it vibrates in all planes perpendicular to the direction of light propagation. The vibrations of the ordinary and the extraordinary rays are perpendicular to each other.
Increasingly, to obtain polarized light, synthetic birefringent substances in the form of the so-called. polaroid plates, operating on the principle of different absorption of two light beams. Calcite deposits have been exploited in Iceland for decades (called, especially in the past, spat or Icelandic spar) exhausted, and others, equally big, not yet discovered. Polaroidowa revelry, from which plates are cut to replace calcite prisms, consists of tiny crystals of herapatite (quinine monosulfate) arranged in parallel in the binder, called nitrocellulose mastic. When a second is applied to the almost transparent foil, but at or near an angle of 90 °, it goes dark, analogous to that observed in a polarizing microscope after the crossing of calcite prisms, the so-called. never.
In optically anisotropic crystals, characterized by double light refraction, there are directions, in which there is no double breakdown. In crystals belonging to the tetragonal system, hexagonal and trigonal has no double refraction towards the main crystal axis. These crystals are called optically uniaxial. Crystals of other systems (rhombic, monoclinic and triclinic) have two such directions, in which there is no double breakdown; they are called optically biaxial. In the direction of the optical axis, light behaves as in optically isotropic bodies, i.e.. in amorphous bodies and in crystals of the regular system.