We use glass all the time, it is practically everywhere. They can be used from enhancing vision, to building bridges. But what is it about glass, that we love it so much? Why is it so important, and why was it revolutionary? Because it is transparent.

This is the most important property of glass. We can see through the glass. Unlike water, though, which we could also see through, glass transformed everything because it was solid. We can shape it into anything we want.

What is transparent?

When we say something is transparent, it means that light can pass through it without being affected (at least to a significant extent). Normally, objects are opaque, meaning that they obstruct light. The light will not be able to pass through them. That is why we can see them, because light reflects off of them and reaches our eyes. But this is not the case with transparent objects. They reflect little light, so it is hard to see them. This is a unique quality and can be used for a lot of things. For example, if we can make it pass through in just the right way, we can ‘bend’ the light to however we wish. This is how glasses work.

In a world much deeper, the quantum world (at the scope of sub-atomic), transparent means that light (photons) pass through the atom without touching or affecting it. In the sub-atomic level, you can say that if the atom was a football field, a pea in the centre would be the nucleus, and electrons are grains of salt from the audience. There is a lot of empty space in between. So photons must pass through the atom if it were to ‘enter’ it. But why doesn’t that always happen? It is quite simple why something is transparent. But what about opaque? Why are most objects what they are, opaque?

Why objects are opaque

The real question we need to ask here is, Why aren’t all objects see-through?. We again go into the quantum world to explain this.

In reality, atoms are not like you learn in the textbooks. Electrons are not in one orbital, or shell all the time. They keep jumping up and down shells. This is because different shells are different ‘energy levels’. Basically, the atom needs more energy to jump to a higher energy level. Atoms at higher shells have more energy. Electrons absorb energy and jump to a higher orbital. Due to a reason still not known properly, they also go down to a lower energy level afterwards.

But there is a catch. Electrons can only absorb specific types of energy in specific amounts. They can only absorb photons (light) as energy to jump to a higher orbital, and light with a specific energy (frequency).

So here is what happens, the electron is waiting to go to a higher energy level and is waiting for a photon, the photon then travels to the atom, the electron absorbs the energy (photon) and then goes up a shell. This means the photon will not be able to travel further, and so the object blocks light.

So every photon can pass through?

Not quite. If the photon passes through the glass or not, depends on the wavelength of it. If it has too much energy, it will not pass through. That is the reason why glass blocks UV light relatively well. Silicon atoms’ electrons do not absorb photons from the visible spectrum because the specific energy required by a photon to be absorbed by the electron is higher. This means that silicon atoms do absorb UV light, which is of higher energy than visible light. This is the reason you will not get a sun-tan that easily when standing in places like telephone booths.

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