Materials. Energy bands. Photons. And why glass is transparent.
1-Minute NomNom
Half empty or half full? As far as glass is concerned, it might be neither. And that is the precise reason behind what makes glass transparent.
Why is this so? Put simply, it all has to do with whether light energy is absorbed by a material or allowed to pass through. This light energy comes in the form of photons, which are the particles that carry light over space. When photons are let through, the material – such as glass – is transparent. When it is absorbed, the material is opaque.
Going deeper into the science
Dive deeper into the atomic level and it becomes even more intriguing. The different electrons surrounding an atom are at different energy levels, also known as energy bands (see diagram below). For electrons to jump to the next higher level (band), the electrons must absorb enough energy. To fall to a lower one, they must lose enough energy.
The amount of energy needed for either is the energy difference between two energy bands, which is also called a band gap. These band gaps are discrete: there must be enough energy absorbed or lost, or the electrons will not change levels (i.e. they do not move halfway or partway). 
This is what determines whether a material is transparent or opaque. When materials have band gaps that can absorb the light energy in photons, these materials – such as the milk cartons above – look opaque when light falls on the materials.
For glass however, the band gaps are larger. Photons do not have sufficient energy to excite the electrons to leap up to the next level. The light simply passes through, giving glass its transparent quality and finally solving the old conundrum: the glass is neither half full nor half empty when it comes to photons and band gaps.
Companies have taken advantage of the transparency of glass to influence how their products are perceived. Top-end bottled water, soda and alcohol brands, for example, sometimes use glass to suggest that the water inside has very high levels of purity and cleanliness.
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