Anisotropy (optical)

refers to the fact that the refractive index n of a crystal depends on the direction of the electric field in the propagating light beam. The velocity of light in a crystal depends on the direction of propagation and on the state of its polarization, i.e. the direction of the electric field. Most noncrystalline materials such as glasses and liquids, and all cubic crystals are optically isotropic, that is the refractive index is the same in all directions. For all classes of crystals excluding cubic structures, the refractive index depends on the propagation direction and the state of polarization. The result of optical anisotropy is that, except along certain special directions, any unpolarized light ray entering such a crystal breaks into two different rays with different polarizations and phase velocities. When we view an image through a calcite crystal, an optically anisotropic crystal, we see two images, each constituted by light of different polarization passing through the crystal, whereas there is only one image through an optically isotropic crystal. Optically anisotropic crystals are called birefringent because an incident light beam may be doubly refracted. Experiments and theories on “most anisotropic crystals”, i.e. those with the highest degree of anisotropy, show that we can describe light propagation in terms of three refractive indices, called principal refractive indices n1, n2 and n3, along three mutually orthogonal directions in the crystal, say x, y and z called principal axes. These indices correspond to the polarization state of the wave along these axes. Crystals that have three distinct principal indices also have two optic axes and are called biaxial crystals. On the other hand, uniaxial crystals have two of their principal indices the same (n1 = n2) and only have one optic axis. Uniaxial crystals, such as quartz, that have n3 > n1 and are called positive, and those such as calcite that have n3 < n1 are called negative A line viewed through a cubic sodium chloride (halite) crystal (optically isotropic) and a calcite crystal (optically anisotropic)

Annealing point of glass

is the temperature at which the viscosity of a glass is about 1012 Pas or (1013 dPa s; dPa is decipascals). For example, for fused silica glass this is about 1180 °C. Viscosity of glass is strongly temperature dependent and decreases steeply as the temperature increases. At the annealing point, the viscosity is sufficient to relieve internal stresses.


Synonyms :
Receiver, Aerial, Ears, Bird snapper, Bullwhip, Rabbit ears, Sky wire, Feelers, Whip, Wire.

A unit that sends or receives radio waves. an antenna is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies an electric current to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves.

Anthony Edward Siegman


Also Known As:




"Anthony (Tony) E. Siegman"

November 23, 1931

Detroit, Michigan

October 7, 2011 (aged 79)
Stanford, California


Electrical engineer, educator concerned with masers and lasers, 

was an electrical engineer and educator concerned with masers and lasers. He was president of the Optical Society of America in 1999 and was awarded the Esther Hoffman Beller Medal in 2009. A prolific scientist and educator, Tony authored more than 200 papers, 20 of which have been cited more than 100 times each. He wrote Microwave Solid-State Masers (1964), An Introduction to Lasers and Masers (1972), and Lasers (1986). This last book, at nearly 1,300 pages, has become the standard reference in the field. In a classic 1961 paper, Tony, together with Bill Louisell and Amnon Yariv, then at Bell Telephone Laboratories, gave the first description of the noise associated with parametric amplification. This led to the discovery (in 1968) of what was then termed parametric fluorescence at optical frequencies. Within 10 years, this type of light source became the standard for experiments with entangled photons, Bell inequalities, and quantum information.












  • Microwave Solid-State Masers, A. E. Siegman (McGraw-Hill, 1964).
  • An Introduction to Lasers and Masers, A. E. Siegman (McGraw-Hill, 1971).
  • Lasers, A. E. Siegman (University Science Books, 1986).
  • Lasers and Their Applications, I. P. Kaminow and A. E. Siegman (eds.) (IEEE Press, Selected Reprint Series, 1973).


Anti-fog coatings prevent reflectors from fogging over in the presence of water vapor. are chemicals that prevent the condensation of water in the form of small droplets on a surface which resemble fog.
The treatments work by minimizing surface tension, resulting in a non-scattering film of water instead of single droplets. This works by altering the degree of wetting. Anti-fog treatments usually work either by application of a surfactant film, or by creating a hydrophilic surface. This assures reliable functioning even in environments charged with water vapor. 

Antireflection coating (AR)

is a thin dielectric layer coated on an optical device or component to reduce the reflection of light and increase the transmitted light intensity. Consider a thin layer of a dielectric material such as Si3N4 (silicon nitride) on the surface of a semiconductor optoelectronic device such as a solar cell. If this antireflection coating has an intermediate refractive index then the thin dielectric coating can reduces the reflected light intensity. In this case n1(air) = 1, n2(coating) ≈ 1.9 and n3(Si) = 3.5. Light is first incident on the air/coating surface and some of it becomes reflected. Suppose that this reflected wave is A. Wave A has experienced a 180° phase change on reflection as this is an external reflection. The wave that enters and travels in the coating then becomes reflected at the coating/semiconductor surface. This wave, say B, also suffers a 180° phase change since n3 > n2. When wave B reaches A, it has suffered a total delay of traversing the thickness d of the coating twice. The phase difference is equivalent to Kc (2d) where Kc=2π/λc is the wavevector in the coating and is given by  2π/λc where λc c is the wavelength in the coating. Since λc = λ /n2 where λ is the free-space wavelength, the phase difference Δφ between A and B is ( 2πn2/λ )(2d). To reduce the reflected light, A and B must interfere destructively and this requires the phase difference to be π or oddmultiples of π, mπ where m = 1,3,5,… is an odd-integer. Thus mπ=2d (2πn2λ)d=m (λ4n2)- Thus, the thickness of the coating must be multiples of the quarter wavelength in the coating and depends on the wavelength. To obtain a good degree of destructive interference between waves A and B, the two amplitudes must be comparable. It turns out that we need n2=(n1n3) When n2=(n1n3) then the reflection coefficient between the air and coating is equal to that between the coating and the semiconductor. In this case we would need (3.5) or 1.87. Thus, Si3N4 is a good choice as an antireflection coating material on Si solar cells. Generally an AR coating operates at one or over a narrow range of wavelengths