Huygens principle of double refraction
In 1669, Rasmus Bartholin made an observation of double refraction in a calcite crystal and documented it in a published work in 1670.
[2] Later, in 1690, Huygens identified polarization as a characteristic of light and provided a demonstration using two identical blocks of calcite placed in succession.
Each crystal divided an incoming ray of light into two, which Huygens referred to as "regular" and "irregular" (in modern terminology: ordinary and extraordinary).
[3] While the Huygens' principle of double refraction explains the phenomenon of double refraction in an optically anisotropic medium, the Huygens–Fresnel principle pertains to the propagation of waves in an optically isotropic medium.
[5] Understanding and forecasting the classical wave propagation of light is based on the Huygens-Fresnel principle.
[3]Electric and magnetic fields that are mutually perpendicular and fluctuating give rise to the transverse electromagnetic wave known as light.
The direction of an electromagnetic wave's electric field vector E is referred to as polarization.
These waves exhibit electric field components that fluctuate at a rapid pace, nearly matching the optical frequency itself, with a time scale of approximately 10−14 seconds.
Light emitted by the sun, incandescent lamps, or candle flames is considered to be unpolarized.
In other words, regardless of the direction in which they are measured, their characteristics, such as optical, electrical, and mechanical, stay constant.
Gases, liquids, and amorphous solids like glass are instances of isotropic materials.
[9] On the other hand, anisotropic materials show various physical characteristics depending on the direction of measurement.
Crystal structure, molecule orientation, or the presence of preferred axes can all be causes of anisotropy.
Crystals, certain polymers, calcite, and numerous minerals are typical examples of anisotropic materials.
The physical characteristics of anisotropic materials, such as refractive index, electrical conductivity, and mechanical qualities, can differ depending on the direction of measurement.
To put it in another way, the light that travels along the optical axis does not experience anisotropic behaviours on the transverse plane.
In these materials, light propagating along the optical axis experience the same effects independently of the polarization.
Light exhibits birefringence within this plane, which means that the refractive index and all the phenomena associated to that, depend on the polarization.
[9] There are two types of uniaxial material depending on the value of index of refraction for the e-ray and o-ray.
On the other hand, the extraordinary ray (E-ray) has an ellipsoidal wavefront due to its refractive index, which varies with the propagation direction within the uniaxial material, leading to different velocities in different directions.
[1]When unpolarized light incidents on the birefringent material, the o-ray and e-ray will generate new wavefronts.
Each plane wavefront propagates straight ahead but with different velocities: V0 for the o-ray and Ve for the e-ray.
However, the Poynting vector, describing the direction of propagation of optical power, is different for the two rays.
