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Once Upon A Day 6 - Why is glass transparent?

I failed the challenge so quickly when it reached Day 4. As to show my determination to continue the challenge, a harder topic is picked for today.

Wood, brick, sand, metals, and many materials are opaque. What is so special about glass that makes it transparent? It turns out to be a bit complicated. But no worries, here is a short video which helped me to grasp the core concepts.

There are a few keywords and phrases mentioned in the video unexplained:

  • 01:00 - "...the edges of the rigidly formed grains..."
  • 01:32 - "...and the result is what is called an amorphous solid..."
  • 02:30 - "...the different energy levels that electrons in an atom can have..."

I googled them and would try to make a little deeper understanding. Follow me if you are interested.

Both glass and sand are made up of silicon dioxide (a.k.a. silica), but why sand is opaque while glass is transparent? It's because of the different moleculer arrangement of the silica.

Sand is a polycrystalline solid which the silica arranges regularly to form crystallites (a.k.a. grains) and pack together very closely to form the sand. When light is shed onto the sand, it's reflected or refracted internally at the grain boundary. Hardly can a light go straight through the sand, making the sand opaque.

On the other hand, glass is amorphous. The molecular arrangement of an amorphous solid is much closer to liquid. When the sand is heated to a few thousands degree celcius, it melts and the silica are freed from the crystallites. At this stage if the sand is cooled rapidly, the silica don't get enough time to crystalline and are "frozen" in the random arrangement. This process is called quenching. Since there is no more grain boundary, light could easily pass through straightly.

(Bonus Video: How Glass is Made)

All amorphous solid such as amorphous metal should be transparent according to this explanation. Obviously it's not the case, but why? The missing piece of information goes to the molecular structure of silica.

Everyone loves freedom, so do electrons. But freedom comes at a cost. The more freedom you want, the more money you'll need. It's the same for electrons, they need energy to gain freedom. When light goes through a material, chances are that its energy is absorted to excite the electrons to a higher energy level. However, electrons are quite picky (傲嬌), they do not accept any amount of energy but the exact amount for them to go to the next energy level. It's just like you cannot go up half step a stair, but a complete step every time.

Because electrons in different materials consist of different sets of energy levels, they absorb light at different wavelengths. For silica, the visible light is probably not their cup of tea. Together as an amorphous solid, glass is transparent to us.

Once Upon A Day: The challenge to write one thing I learned for the day.