Transparency and translucency


In the field of optics, transparency is the physical property of allowing light to pass through the material without appreciable scattering of light. On a macroscopic scale, the photons can be said to follow Snell's law. Translucency is the physical property of allowing light to pass through the material. It allows light to pass through but the light does not necessarily follow Snell's law on the macroscopic scale; the photons may be scattered at either of the two interfaces, or internally, where there is a change in the index of refraction. In other words, a translucent material is made up of components with different indices of refraction, while a transparent material is made up of components with a uniform index of refraction. Transparent materials appear clear, with the overall appearance of one color, or any combination leading up to a brilliant spectrum of every color. The opposite property of translucency is opacity. Other categories of visual appearance, related to the perception of regular or diffuse reflection and transmission of light, have been organized under the concept of cesia in an order system with three variables, including transparency, translucency and opacity among the involved aspects.
When light encounters a material, it can interact with it in several different ways. These interactions depend on the wavelength of the light and the nature of the material. Photons interact with an object by some combination of reflection, absorption and transmission.
Some materials, such as plate glass and clean water, transmit much of the light that falls on them and reflect little of it; such materials are called optically transparent. Many liquids and aqueous solutions are highly transparent. Absence of structural defects and molecular structure of most liquids are mostly responsible for excellent optical transmission.
Materials that do not transmit light are called opaque. Many such substances have a chemical composition which includes what are referred to as absorption centers. Many substances are selective in their absorption of white light frequencies. They absorb certain portions of the visible spectrum while reflecting others. The frequencies of the spectrum which are not absorbed are either reflected or transmitted for our physical observation. This is what gives rise to color. The attenuation of light of all frequencies and wavelengths is due to the combined mechanisms of absorption and scattering.
Transparency can provide almost perfect camouflage for animals able to achieve it. This is easier in dimly-lit or turbid seawater than in good illumination. Many marine animals such as jellyfish are highly transparent.

Etymology

  • late Middle English: from Old French, from medieval Latin transparent- 'visible through', from Latin transparere, from trans- 'through' + parere 'be visible'.
  • late 16th century : from Latin translucent- 'shining through', from the verb translucere, from trans- 'through' + lucere 'to shine'.
  • late Middle English opake, from Latin opacus 'darkened'. The current spelling has been influenced by the French form.

    Introduction

With regard to the absorption of light, primary material considerations include:
  • At the electronic level, absorption in the ultraviolet and visible portions of the spectrum depends on whether the electron orbitals are spaced such that electrons can absorb a quantum of light of a specific frequency. For example, in most glasses, electrons have no available energy levels above them in the range of that associated with visible light, or if they do, the transition to them would violate selection rules, meaning there is no appreciable absorption in pure glasses, making them ideal transparent materials for windows in buildings.
  • At the atomic or molecular level, physical absorption in the infrared portion of the spectrum depends on the frequencies of atomic or molecular vibrations or chemical bonds, and on selection rules. Nitrogen and oxygen are not greenhouse gases because there is no molecular dipole moment.
With regard to the scattering of light, the most critical factor is the length scale of any or all of these structural features relative to the wavelength of the light being scattered. Primary material considerations include:
  • Crystalline structure: whether the atoms or molecules exhibit the 'long-range order' evidenced in crystalline solids.
  • Glassy structure: Scattering centers include fluctuations in density or composition.
  • Microstructure: Scattering centers include internal surfaces such as grain boundaries, crystallographic defects, and microscopic pores.
  • Organic materials: Scattering centers include fiber and cell structures and boundaries.
Diffuse reflection - Generally, when light strikes the surface of a solid material, it bounces off in all directions due to multiple reflections by the microscopic irregularities inside the material, and by its surface, if it is rough. Diffuse reflection is typically characterized by omni-directional reflection angles. Most of the objects visible to the naked eye are identified via diffuse reflection. Another term commonly used for this type of reflection is "light scattering". Light scattering from the surfaces of objects is our primary mechanism of physical observation.
Light scattering in liquids and solids depends on the wavelength of the light being scattered. Limits to spatial scales of visibility therefore arise, depending on the frequency of the light wave and the physical dimension of the scattering center. Visible light has a wavelength scale on the order of 0.5 μm. Scattering centers as small as 1 μm have been observed directly in the light microscope.

Transparent ceramics

Optical transparency in polycrystalline materials is limited by the amount of light scattered by their microstructural features. Light scattering depends on the wavelength of the light. Limits to spatial scales of visibility therefore arise, depending on the frequency of the light wave and the physical dimension of the scattering center. For example, since visible light has a wavelength scale on the order of a micrometre, scattering centers will have dimensions on a similar spatial scale. Primary scattering centers in polycrystalline materials include microstructural defects such as pores and grain boundaries. In addition to pores, most of the interfaces in a typical metal or ceramic object are in the form of grain boundaries, which separate tiny regions of crystalline order. When the size of the scattering center is reduced below the size of the wavelength of the light being scattered, the scattering no longer occurs to any significant extent.
In the formation of polycrystalline materials the size of the crystalline grains is determined largely by the size of the crystalline particles present in the raw material during formation of the object. Moreover, the size of the grain boundaries scales directly with particle size. Thus, a reduction of the original particle size well below the wavelength of visible light eliminates much of the light scattering, resulting in a translucent or even transparent material.
Computer modeling of light transmission through translucent ceramic alumina has shown that microscopic pores trapped near grain boundaries act as primary scattering centers. The volume fraction of porosity had to be reduced below 1% for high-quality optical transmission. This goal has been readily accomplished and amply demonstrated in laboratories and research facilities worldwide using the emerging chemical processing methods encompassed by the methods of sol-gel chemistry and nanotechnology.
Image:Backlit mushroom.jpg|thumb|Translucency of a material being used to highlight the structure of a mushroom
Transparent ceramics have created interest in their applications for high energy lasers, transparent armor windows, nose cones for heat seeking missiles, radiation detectors for non-destructive testing, high energy physics, space exploration, security and medical imaging applications. Large laser elements made from transparent ceramics can be produced at a relatively low cost. These components are free of internal stress or intrinsic birefringence, and allow relatively large doping levels or optimized custom-designed doping profiles. This makes ceramic laser elements particularly important for high-energy lasers.
The development of transparent panel products will have other potential advanced applications including high strength, impact-resistant materials that can be used for domestic windows and skylights. Perhaps more important is that walls and other applications will have improved overall strength, especially for high-shear conditions found in high seismic and wind exposures. If the expected improvements in mechanical properties bear out, the traditional limits seen on glazing areas in today's building codes could quickly become outdated if the window area actually contributes to the shear resistance of the wall.
Currently available infrared transparent materials typically exhibit a trade-off between optical performance, mechanical strength and price. For example, sapphire is very strong, but it is expensive and lacks full transparency throughout the 3–5 μm mid-infrared range. Yttria is fully transparent from 3–5 μm, but lacks sufficient strength, hardness, and thermal shock resistance for high-performance aerospace applications. A combination of these two materials in the form of the yttrium aluminium garnet is one of the top performers in the field.

Absorption of light in solids

When light strikes an object, it usually has not just a single frequency but many. Objects have a tendency to selectively absorb, reflect, or transmit light of certain frequencies. That is, one object might reflect green light while absorbing all other frequencies of visible light. Another object might selectively transmit blue light while absorbing all other frequencies of visible light. The manner in which visible light interacts with an object is dependent upon the frequency of the light, the nature of the atoms in the object, and often, the nature of the electrons in the atoms of the object.
Some materials allow much of the light that falls on them to be transmitted through the material without being reflected. Materials that allow the transmission of light waves through them are called optically transparent. Chemically pure window glass and clean river or spring water are prime examples of this.
Materials that do not allow the transmission of any light wave frequencies are called opaque. Such substances may have a chemical composition which includes what are referred to as absorption centers. Most materials are composed of materials that are selective in their absorption of light frequencies. Thus they absorb only certain portions of the visible spectrum. The frequencies of the spectrum which are not absorbed are either reflected back or transmitted for our physical observation. In the visible portion of the spectrum, this is what gives rise to color.
Absorption centers are largely responsible for the appearance of specific wavelengths of visible light all around us. Moving from longer to shorter wavelengths: Red, orange, yellow, green, and blue can all be identified by our senses in the appearance of color by the selective absorption of specific light wave frequencies. Mechanisms of selective light wave absorption include:
  • Electronic: Transitions in electron energy levels within the atom. These transitions are typically in the ultraviolet and/or visible portions of the spectrum.
  • Vibrational: Resonance in atomic/molecular vibrational modes. These transitions are typically in the infrared portion of the spectrum.