In the field of optics,transparencyIt is the physical property that allows light to pass through the material without dispersion.At the macro scale (where the size studied is much larger than the wavelength of the photon discussed), the photon followsSnell Law 。Translucency is transparentSuperset: It allows light to pass through, but does not necessarily conform to Snell's law;Photons can be dispersed in either of the two interfaces.In other words, the translucent medium allows the transmission of light, while the transparent medium not only allows the transmission of light, but also allows image formation.The opposite property of transparency is opacity.Transparent materialIt looks clear, and the overall appearance is one color, or any combination, to generate the spectrum of each color.
In the field of optics, transparency is a physical property that allows light to pass through materials without being dispersed.At the macro scale (where the size studied is much larger than the wavelength of the photon discussed), the photon followsSnell Law 。Translucency is transparentSuperset: It allows light to pass through, but does not necessarily conform to Snell's law;Photons can be dispersed in either of the two interfaces.In other words, the translucent medium allows the transmission of light, while the transparent medium not only allows the transmission of light, but also allows image formation.The opposite property of transparency is opacity.Transparent materialIt looks clear, and the overall appearance is one color, or any combination, to generate the spectrum of each color.
When light encounters a material, it can interact with it in several different ways.These interactions depend on the wavelength of light and the nature of the material.Photons interact with objects through some combination of reflection, absorption and transmission.Some materials, such asplate glassAnd clean water, will most of the light transmitted to them, almost no reflection, this material is called optically transparent.Many liquids and aqueous solutions are highly transparent.The structural defects (cavities, cracks, etc.) and molecular structures of most liquids are the main reasons for optical transmission.
The opaque material is called noTransparent material。Many of these substances have a chemical composition, including substances called absorption centers.Many substances absorb white light frequency selectively.They absorb the visible spectrum of a specific part while reflecting other parts.The frequency of unabsorbed spectrum can be reflected or transmitted through our physical observation.This is the reason for color.The attenuation of light at all frequencies and wavelengths is due to the combined mechanism of absorption and scattering.
Transparency can provide perfect camouflage for animals.In dim or turbid sea water, this is easier than in good light.manymarine animal, such as jellyfish, are highly transparent.[1]
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The absorption of light mainly includes:
(1) At the electronic level, the absorption of the ultraviolet and visible light (UV Vis) parts of the spectrum depends on whether the electronic orbits are quantified, so that they can absorb specific light (or photon) quantum frequencies without violating the selection rules.For example, in most glasses, electrons have no available energy level in the range related to visible light, or if they do so, they violate the selection rule, which means that there is no obvious absorption in pure (undoped) glass, making them ideal for building windowsTransparent material。
(2) At the atomic or molecular level, the physical absorption of the infrared part of the spectrum depends on the frequency of atomic or molecular vibration or chemical bond, as well as the selection rules.nitrogenandoxygenIt is not a greenhouse gas, because there is no molecular dipole moment.
With regard to light scattering, the most critical factor is the scale of all these structural features relative to the wavelength of the scattered light.It mainly includes:
(1) Crystal structure: whether atoms or molecules show the "long-term order" in crystal solids.
(2) Glass structure: The scattering center includes the fluctuation of density or composition.
(3) Microstructure: Scattering centers include internal surfaces, such as grain boundaries, crystal defects, and micropores.
(4) Organic materials: Scattering centers include fiber and cell structures and boundaries.
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An object may be opaque because it reflects incident light or absorbs incoming light.Almost all solids reflect and absorb part of the incident light.
When light falls on a metal block, it encounters atoms tightly packed in a regular lattice, and "electrons between atoms" move randomly.In metals, most of them are non bonding electrons (or free electrons), which is contrary to the bonding electrons usually found in covalent bonding or ionic bonding non-metallic (insulating) solids.In metal bonds, atoms in the crystal structure may easily lose any potential bonding electrons.This effect of delocalization only exaggerates the role of the "Sea of Electronics".As a result of these electrons, the incident light in most metals is reflected back, which is why we see the shiny metal surface.
Most insulators (or dielectric materials) are held together by ionic bonds.Therefore, these materials do not have free conduction electrons, and the bonding electrons only reflect a small part of the incident wave.The remaining frequency (or wavelength) can be propagated (or transmitted) freely.These materials include all ceramics and glasses.
If the dielectric material does not contain light absorbing additive molecules (pigments, dyes, colorants), it is usually transparent to the spectrum of visible light.The color center (or dye molecule or "dopant") in the dielectric absorbs part of the incident light.The remaining frequency (or wavelength) can be reflected or transmitted freely.This is it.Stained glassProduction mode.
Most liquids and aqueous solutions are highly transparent.For example, water, cooking oil, wine, air and natural gas are transparent.The absence of structural defects (voids, cracks, etc.) and the molecular structure of most liquids are mainly due to their excellent optical transmission.The ability of liquid to flow through viscous "internal" defects is one of the reasons why some fibrous materials (such as paper or fabric) increase their apparent transparency when wetted.The liquid fills many voids, making the material more uniform in structure.
Light scattering in an ideal defect free crystalline (non-metallic) solid that does not provide a scattering center for incident light will be mainly due to any influence of non harmonics in the ordered lattice.Because of the typical crystalline materialanisotropy, including their symmetry group and Bravis lattice, the light transmission will be highly oriented.For example, seven different crystal forms of quartzsilicon dioxide(Silicon dioxide, silicon dioxide)Transparent material。
In optical waveguide
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Optical transparent materials focus on the response of materials to incident light waves in the wavelength range.Guided light wave transmission through frequency selective waveguides involves the emerging field of optical fibers, and the frequency range (multimode fiber) in which some glass compositions are used as transmission media at the same time, with little or no interference between wavelengths.This resonance mode of energy and data transmission through electromagnetic (optical) waves is relatively lossless.
An optical fiber is a cylindrical dielectric waveguide that propagates light along its axis through the process of total internal reflection.The fiber consists of a core surrounded by a cladding.In order to limit the optical signal in the core, the refractive index of the core must be greater than that of the cladding.The refractive index is a parameter reflecting the speed of light in the material.(The refractive index is the ratio of the speed of light in vacuum to the speed of light in a given medium, so the refractive index of vacuum is 1.) The greater the refractive index, the slower the light travels in the medium.The typical values of core and cladding of optical fiber are 1.48 and 1.46 respectively.
When the light traveling in the dense medium hits the boundary at a steep angle, the light will be completely reflected.This effect, called total internal reflection, is used in optical fibers to confine light to the core.Light bounces repeatedly from the boundary along the fiber.Because light must hit the boundary at an angle greater than the critical angle, only light entering the fiber within a certain angle range will be propagated.The angle range is called the receiving cone of the fiber.The size of the receiving cone is a function of the refractive index difference between the fiber core and the cladding.Optical waveguides are used as components of integrated optical circuits (e.g., combined with lasers or light-emitting diodes) or as transmission media in local and long-distance optical communication systems.
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Many floating near the surfacemarine animalHighly transparent, giving them almost perfect camouflage.However, the transparency of an object made of a material with a refractive index different from that of sea water is difficult.Some marine animals, such as jellyfish, have a gel mainly composed of water;Their dense mesogloea is cell-free and highly transparent.This makes them float, but it also makes their muscles large, so they can't swim fast. This form of camouflage becomes a costly mobility trade-off.The transparency of gelatin zooplankton is 50-90%.50% transparency is enough to make animals invisible to predators such as cod, with a depth of 650 meters (2130 feet);In shallow waters, the light is brighter, and predators can see better invisibility, requiring better transparency.For example, cod can see 98% transparent prey in the best illumination in shallow water.Therefore, it is easier to achieve sufficient transparency of camouflage in deeper waters.For the same reason, air transparency is more difficult to achieve, but an example of translucent skin and pale green limbs was found in the glass frog in the rainforest of South America.[2]
