Glass can be both transparent and reflective because its surface can act as a mirror when treated with special coatings, while still allowing light to pass through its amorphous mass.
Glass is mainly composed of silica, which is essentially melted sand. When it cools, this material forms a somewhat special molecular structure called amorphous structure, unlike crystals that organize their atoms in an ordered manner. In glass, it’s a joyful molecular jumble without precise regularity. Exactly because the atoms are disordered, light rays pass through quietly without being too obstructed. This disorder makes glass transparent, allowing light to pass directly through without too many internal reflections. If it were a classic crystal, with its regular rows of atoms, light would tend to bounce or be otherwise absorbed, making it opaque or colored. The absence of absorbing particles and the simplicity of its composition also contribute to its usual transparency.
When light hits a glass surface, a portion of the rays is directly reflected: this is reflection. This phenomenon occurs because light encounters a sudden change of medium (air-glass), causing a partial bounce. The rest of the rays continues its path through the glass but is slightly deflected: this is called refraction. Glass has a different density than air, so the speed of light slows down when passing through it, resulting in a change of direction. This duo of reflection-refraction explains why we can clearly perceive through the windows while also distinguishing our own reflection, especially when the outside is dark or the glass is illuminated.
Raw glass only reflects a little bit of light, but by applying certain treatments to its surface, you can completely change the game. For example, an anti-reflective treatment significantly reduces reflection by creating a special layer that forces light rays to cancel out their own reflections. The result: the glass becomes even more transparent and you can see better through it. Conversely, a reflective treatment (like that of mirrored sunglasses or one-way mirrors) creates a thin metallic layer on the surface, which reflects a large portion of the light rays, providing a pronounced mirror effect on one side while remaining transparent on the other. It's all a matter of thickness and the material used in these thin layers; just a few nanometers are enough to completely transform how glass interacts with light.
When you look out the window during the day, you see through it clearly; however, in the evening, as soon as the light inside is turned on, it becomes a true mirror, perfectly reflecting your face. The same thing happens with the screen of a smartphone or tablet: when the screen is off, it acts like a small mirror, while as soon as it is turned on, you can see clearly through the glass to the LCD display. The same phenomenon occurs with sunglasses: from your side, you can clearly observe the surroundings, while others facing you often see only a reflection of themselves or the scenery. Store windows play precisely on this; depending on how they are lit, they can either show their contents or reflect the outside for aesthetic or practical effects. These everyday examples illustrate how much the dual property of transparency and reflection mainly depends on differences in lighting, treatments applied to the glass, and the specific context in which it is viewed.
Ordinary glass slightly absorbs blue-green light, which explains why the edge of a thick glass often shows a pale green tint.
Modern mirrors are not made up solely of glass; they combine a thin metallic layer (silver or aluminum) coated with a protective layer applied to the back of the glass to ensure optimal and long-lasting reflection.
Some buildings have windows treated with an invisible metallic coating called 'low-emissivity coating', which ensures excellent transparency while reflecting solar heat and thereby reducing the greenhouse effect indoors.
The phenomenon of a bird hitting a window can be explained by the glass's ability to reflect some of the outside light. To avoid this issue, there are specific films or stickers that are nearly transparent to humans but visible to birds.
This transparency comes from the molecular structure of glass, which allows visible light to pass through without being significantly absorbed or scattered. Unlike materials such as metal or wood, which absorb or scatter light more, glass freely permits the passage of the visible spectrum.
To reduce reflection, special treatments known as anti-reflective coatings are usually applied directly to the surface of the glass. These treatments significantly decrease the amount of light reflected, thereby improving perceived transparency.
Tinted glass generally loses some transparency, as the pigments added to the material absorb specific parts of the light spectrum. However, it can still reflect light under certain conditions, such as lighting and viewing angles.
Even though the glass is transparent, part of the light that reaches its surface is reflected, creating a mirror-like image. The darker the room behind the glass, the more pronounced the reflection effect, making your own reflection visible.
Most commonly used glasses indeed possess these two characteristics. However, certain types of specially treated glass, such as anti-reflective or tinted glasses, may have less reflection or reduced transparency.
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