Glass is translucent because its atomic structure allows it to let a significant amount of light pass through, while metals have a crystalline structure that absorbs light, making them opaque.
Glass is primarily a material that is classified as amorphous solid. This means that its atoms are arranged in a rather disordered manner, as if they had been frozen in mid-motion, much like a snapshot taken in real time. In contrast, metal generally has a crystalline structure: its atoms neatly align in a regular and periodic order, almost like soldiers in a military parade. This organization greatly affects how these materials interact with light. The apparent chaos of glass allows light to pass through its structure smoothly, whereas the strict atomic regularity of metal, with its free electrons, effectively blocks the passage of light rays.
When light passes through a material, the interactions of photons with electrons are crucial. In the case of glass, the electrons are tightly bound to the atoms. As a result, when a photon arrives, these electrons cannot easily absorb or reflect it, so they let the light pass through them with almost no interaction. This is what makes glass translucent. In contrast, in a metal, the electrons are relatively free, able to move easily from one atom to another. When these mobile electrons encounter a photon, they quickly absorb or reflect it, thus preventing light from passing through. It is this phenomenon, directly related to the mobility of the electrons, that gives metal its shiny and opaque appearance.
Electrical conductivity mainly comes from free electrons, those electrons capable of moving easily between the atoms of a material. In metal, there are so many free electrons that a light wave trying to pass through quickly loses its energy: it is either directly absorbed or reflected at the surface. As a result, the metal appears opaque and shiny. In glass, on the other hand, there are few or no free electrons to stop the light rays. They pass through smoothly because they do not have to face these battalions of mobile electrons blocking their way. This is why glass remains translucent, while metal lets nothing through.
Each photon, this tiny particle of light, carries a certain amount of energy. This energy depends on the color: blue and ultraviolet photons are highly energetic; red or infrared photons are less charged. When these photons travel through a material, they may encounter electrons. In metal, these electrons are numerous and hyperactive, ready to absorb almost all light energies and re-emit them, which makes the metal opaque and shiny. In glass, it's different: its electrons only effectively capture very energetic photons, like ultraviolet ones, and allow most visible light photons to pass through quietly. The result: glass remains translucent, letting significantly more light through with little absorption or reflection.
Structural defects in a material are somewhat like bumps on a pane of glass: they change the way light passes through or bounces off. In the case of glass, these defects are limited or too small to seriously impede light, making it fairly clear to the eye. In contrast, defects in metals—irregular grains, impurities, or dislocations—scatter light significantly. This intense scattering sharply disrupts the passage of light and renders the metal opaque. A material with few defects is often more transparent. The more bumps and structural imperfections there are, the more muddled it becomes for the photons trying to pass through: as a result, less transparency.
Opaque glasses used in decoration often contain a large number of internal defects or finely dispersed crystals, which strongly diffuse light rather than allowing it to pass freely.
Did you know that silicon, mainly used to make electronic components, can resemble a shiny metal but becomes translucent or transparent in thin form? This paradox is linked to its unique semiconductor status.
Some metals can become translucent or even transparent when they are made very thin (a few nanometers thick). For example, ultra-thin gold foil appears semi-transparent and takes on a surprising blue-green color.
The color of glass can vary greatly, depending on impurities or the intentional addition of other chemical elements. For example, the green glass of traditional bottles is due to the presence of ferrous oxide.
At high temperatures, the internal structure of the material can change, creating more atomic irregularities and structural imperfections that scatter light randomly, thus leading to a gradual transition to opacity.
Colored glass is generally translucent or even transparent, depending on the pigments used. Some metal ions present in the glass absorb specific frequencies of light, imparting color while allowing part of the light spectrum to pass through.
In general, no, but some metals like gold can become partially transparent when stretched into extremely thin sheets (gold leaf a few nanometers thick), allowing a very limited amount of light to pass through due to specific quantum phenomena.
Impurities, such as certain metallic ions or structural contaminants, can selectively absorb or deviate certain wavelengths of light, thereby reducing the material's transparency and giving it a cloudy or colored appearance.
The glass has an amorphous atomic structure that allows photons to pass through with minimal interactions, whereas metal contains a sea of free electrons that react strongly with light, thus blocking light transmission even through a very thin layer.
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