§4 Polarizing prisms and polaroids
is a device by which the natural light of the possible, to get a plane
polarized light. The basis of the polarization devices based on the
phenomenon of birefringence.
Polarization devices are divided into the prism and polaroids. Prism
divided into polarizing prism (giving one plane polarized beam) and a
birefringent prism (two polarized rays in mutually perpendicular
using the principle of total internal reflection of the usual beam.
Total internal reflection occurs when light is incident on the boundary
between an optically denser medium to an optically less dense medium.
At angles greater than the critical icr light is
totally reflected, not refracted. The intensity of the reflected light
in this case is equal to the intensity of the incident light.
The prism (Scottish scientist 1768-1851) is a double prism of Iceland spar, glued along the line AB Canada balsam, with n = 1.55. The optical axis OO' prism of the inlet face angle 48°. On the front face of the prism natural beam, parallel to the edge CB splits into two beams: the ordinary (nO = 1.86) and extraordinary (ne
= 1.51). An appropriate choice of the angle of incidence equal to or
greater limiting (critical) ordinary ray undergoes total internal
reflection (as Canada balsam for him - the less dense medium), and then
is absorbed by the blackened surface of the CB. Extraordinary
beam exits the crystal parallel to the incident beam slightly offset
relative to the incident (due to refraction at the edges AC and BD).
prism using the difference in the refractive indices of the ordinary
and extraordinary rays in order to separate them as far as possible from
each other. They are made of Iceland spar and glass, Iceland spar
prisms with perpendicular optic axes.
Birefringent crystals have dichroism (di - two, chrome - color) - the absorption of light depends on the orientation ,
and the direction
of light propagation in the crystal and the wavelength. Dichroism
phenomenon manifests itself in different colors of crystals in different
directions. An example of a dichroic crystal tourmaline - uniaxial
crystal, which is absorbed by the ordinary ray is much more unusual.
Even more pronounced dichroism have crystals herapathite (quinine sulfate iodine). Herapathite film thickness of about ~ 0.1 mm
fully absorbs the rays of ordinary visible light. Dichroic crystals
are used for the manufacture of polaroids - thin crystalline films,
allowing to obtain a plane polarized light.
§5 The analysis of polarized light
Plane-polarized light within the crystal plate is divided into ordinary
and extraordinary. On exit from the rays of the plate will be formed
and give elliptically polarized light:
Between ordinary and extraordinary rays in the plate there path difference
or the phase difference
called a quarter-wave plate (plate ) (“+”for positive crystals , “-”for negative). If , at the output of the equation
If the light is incident on the plate at an angle α = 45°, then Е0 = Ееand the output is circularly polarized light. The plate, which
called a half-wave plate.
§ 6 Artificial optical anisotropy
Several crystals are isotropic, as a result of external influence becomes optically anisotropic. Artificial anisotropy can cause:
(compression or tension) cubic crystals, glasses and other amorphous
solids. In all cases, external influences substance acquires the
properties of a uniaxial crystal (ie becomes birefringent), the optical
axis coincides with the direction of strain, electric and magnetic
where σ - elastic deformation.
k1 - a constant characterizing the material.
When F = 0 light on the screen does not work. When F ≠ 0 on the screen there is an interference pattern.
2. electric field (Kerr effect - the emergence of artificial optical
anisotropy of the electric field in liquids, gases, amorphous solids);
where E - electric field,
k2 - a constant characterizing the material.
3. magnetic field (Cotton-Mouton) the emergence of artificial optical
anisotropy in the magnetic field in liquids, glasses, and colloids. A
measure of the emerging optical anisotropy is the difference of the
refractive indices of the ordinary and extraordinary rays in the
direction perpendicular to the optical axis.
where H - the magnetic field,
k3 - a constant characterizing the material.