Friday December 23, 2011 Q&A

Why is glass see-through?


1 Because its atoms are randomly arranged

2 Due to its quantum properties

3 Due to a component added to the sand

Question related image
Answer: Due to its quantum properties. However, part of the answer is also indirectly linked to the atomic structure of glass. In opaque bodies, light particles – photons – interact with the atoms’ electrons and are absorbed. In materials which let daylight through, the atoms allow almost all photons to pass through. How does this work? In the case of glass, a frequent explanation contends that glass is an amorphous material, i.e. with a disorderly atomic structure. This disorder is supposed to create gaps and paths through which light can easily pass. “Yet atomic order has nothing to do with this, as extremely well-ordered materials such as diamonds are also transparent”, explains Libero Zuppiroli, Professor at the EPFL Laboratory of Optoelectronics of Molecular Materials. Another approach is based on the insulating properties of glass: based on silicon dioxide, glass is an excellent electrical insulator. The theory here is that light cannot move electrons within the glass, and is therefore able to pass through without interaction. Again, this is not the right answer as a number of see-through conducting materials are commonly used in optoelectronics. The forbidden band “Only quantum mechanics can provide a satisfactory answer – but this is entirely beyond our intuition”, says the scientist. According to what physicists call band theory, materials have various energy levels, some of which totally exclude electrons. These so-called band gaps vary from one material to the next. In glass, these band gaps between two energy levels are particularly large. Thus a photon of visible light which hits an electron in glass does not provide it with enough energy to skip from one energy band to another. Consequently, photons are not absorbed and do not lose their energy, and visible light travels through glass unhampered. These quantum properties of glass explain not only the material’s transparency, but also its refractive power. Libero Zuppiroli deals with the subject in depth in his book entitled “Traité des couleurs”.