Transparency and translucency

Transparency (from the Latin trans " (back ) by" and parere " show seem " ) is in physics, the ability of matter, electromagnetic waves pass therethrough (transmission). In everyday life, the term is usually applied to light, so the visible to the human spectrum of electromagnetic radiation related.

In line with this, a distinction is made in the Seegangshydrodynamik or offshore engineering between hydrodynamically transparent structures which let surface waves, and hydrodynamically compact structures that reflect surface waves.

Fundamentals and disambiguation

Transparency is an optical property of a material; other optical properties such as the reflectivity and absorptivity. The optical properties of materials are closely related to the electrical properties of a material, for example the presence of free electrons or the band structure. If a material for incident electromagnetic radiation (photons ) of a more or less broad spectrum transparent, it can fully penetrate the material almost, is thus hardly reflected and hardly absorbed.

In everyday life, a material such as window glass, transparent or translucent called when you can see behind Lying relatively clear, so the material is substantially transparent to radiation in the visible spectrum.

Incident photons interact with different components depending on the energy of the material, thus the transparency of a material is dependent on the frequency of the electromagnetic wave. Materials which are opaque to light, may be transparent to other wavelengths of the electromagnetic spectrum, such as x-rays. In the field of infrared radiation are, for example, the vibrational energies of molecules or groups of molecules or even the free electrons in the electron gas of metals. In the visible region, the energy of the photons in the range of the binding energy of valence electrons, this can be excited by absorption of a photon in the conduction band. The involved photon is completely " wiped out ". If a majority of the photons are absorbed, a material is opaque ( subsequent effects such as recombination are neglected here first ). The band structure of the material is thus (among others) decisive for its transparency.

It is important in the absorption of photons, that this is done only in certain " energy portions " ( quantum). That is, only photons of a given energy can be absorbed photons with a higher or lower energy are unaffected. Insulating materials such as glass are generally transparent because its band gap is greater than the photon energy for visible light. This photon therefore can not be absorbed by valence. The reason for this lies in the band structure of the material is influenced to each other, for example, by the distance between the atoms. Not excited at that glass the valence electrons in the conduction band and thus not available for the charge transport available, further causes the glass is not electrically conductive. Semiconductors having a smaller energy band gap, on the other hand higher energy photons are absorbed ( blue light). From the overall appearance, these materials are not transparent, therefore, even if they are transparent seen, for example, red light. From the pure spectral transparency, however, the color impression can not be derived directly.

Mere light transmission such as in milk glass is not critical in general, to be designated as transparent. With frosted glass the light is scattered by a rough surface or by particles in the material. The case transmitted light is called diffuse light, since no sharp image located behind objects are. If only darker and lighter areas visible, one speaks of translucency. In weakly translucent materials, the transparent property held as translucency is specified as opacity. Low light is a superficial translucency.

Occurrence

Transparency is usually added in gaseous materials ( e.g. air ), but also for some liquids and solids, such as clear water, ordinary glass, and some plastics. If the degree of transparency depends on the wavelength of light, then the transparent medium is tinted. This may in certain metal oxide molecules in the glass or (larger) colored particles, such as in colored smoke, lie. Are many of these colored particles present, the gas, the liquid or solid is opaque, such as thick smoke.

Because transparent matter

Conditional transparencies are phototropy and Elektrotropie.

Phototropy

Photochromic glass is transparent glass, which responds to UV light. It is also referred to as self-tinting. The phototropism is based on a reversible transformation of stored silver halide- precipitates. In the process, the glass is colored. Depending on the glass Halogenidart different colors can be produced. Brown or gray photochromic lenses are used for the manufacture of sunglasses that are transparent again in bright light by itself ( quickly ) and darker with decreasing brightness (slower). The difference in speed is due to the fact that a balance of two opposing reactions sets: The nightfall runs in a reaction 0th order ( each incident, appropriate in the wavelength photon causes a molecule conversion ). In contrast, the reverse process is dependent on the temperature of 1st order reaction, which proceeds ( respond in equal times equal shares, cf half-life ) to an exponential function. These properties have the effect that such glasses are not so good for driving a car when the brightness changes in quick succession, at a tunnel entrance glasses remains (too) dark long. In cold and high brightness - in winter snow - the black glasses; began to realize she is in the dark, quickly gets under warm water.

Phototropy also plays a role in photosynthesis.

Elektrotropie

Elektrotropes glass is a form of glass which, although translucent but opaque ( opaque, similar to frosted glass ) in the normal state and only by applying electrical voltage is transparent. This is done by means of liquid crystals, which are located between two layers of normal glass. Technically, these discs work similarly to an LCD display. Man sets elektrotropes glass as a switchable privacy glass. The user can decide when you can look through the glass and when not to. Examples are glass partitions in sedans (eg, Maybach 57 and 62) and the cutting discs between cab and panoramic compartment ( "Lounge" ) in the end car of the ICE 3 and ICE T. Elektrotrope lenses are among the so-called intelligent glasses.