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Why is glass transparent?

drinking glassThis is a very interesting question. What else can you think of that is transparent? Are they liquids, gases or solids? And what do we mean when we say that something is transparent? That we can see through it, I’m guessing is your answer, but it’s not the full answer. The energy that we call light is actually only part of a whole spectrum of energies called the electromagnetic spectrum. This spectrum includes all sorts of energy such as radio waves, microwaves, X-rays and even radioactive rays, as well as the light we can see. So, materials can be transparent to one type of energy but not to another, and glass is no exception, we can see through it but it is not transparent to X-rays. So, what’s the difference? How does the energy arriving at the material know what to do? ReflectionWell, let’s look at the options for a packet of energy arriving at the surface of a material (these packets of energy are called photons). The first option is reflection; you’ve all looked in a mirror so you know how this works, the energy bounces off the surface, and, if it’s visible light, you’ll get an image, your face in the mirror, trees reflected in a lake. If this didn’t happen we really wouldn’t be able to see very much, other than sources of light, like the sun. Try looking at your face in a window; you’ll see that even glass is slightly reflective.

What’s next? Well, if the energy doesn’t bounce of the surface, it goes into the material and can either come out the other side, or be absorbed by the material. This absorption means the energy is transferred from the photon into the material, which causes the material to change in some way, usually to heat up. As with reflection, different materials absorb different energies of light, so some bounce off, and some are transferred to the material; this makes things appear to be different colours (we refer to the different energies of visible light as colours!). So, if a material reflects all the colours, it looks white, if it absorbs all the colours, it looks black. A good test of how much energy a material is absorbing is how much it heats up if you leave it in the sun.

windowThe third option would be for the photon to go into the material and come out the other side, and this is what we would call transparency. But is it really that simple? Does the photon not interact with the material at all? Does it just pass straight through without changing? Well, no, it’s not that simple, especially in solids, there are no big holes that allow the light to just pass straight through, so what does happen? Well, we already know that different materials behave differently with different types of energy, so maybe what the material is made of is the answer.

Remember all the examples of transparent materials you thought of before? Hopefully one of them was diamond, which is nice and transparent, and also hopefully, graphite wasn’t one of your examples, because it isn’t transparent at all, not to visible light anyway. That’s strange though, aren’t diamond and graphite both made out of carbon? These two materials, both made entirely of carbon have widely different material properties and this is because of how the atoms are arranged; this is known as the lattice structure of the material. So, two materials with the same atoms, but a different lattice structure, can be the hardest substance known to man, or one of the softest. The arrangement of the atoms within a material allows the lattice to vibrate in certain modes, called phonons, which represent discrete ways in which the lattice can move. These phonons determine a number of the material’s properties, including transparency.

So the question now becomes not ‘why is a material transparent?’ but ‘when is material transparent?’. If the structure of the material allows a photon to be absorbed and turned into heat, the material isn’t transparent, but if the structure allows the photon to be absorbed and re-emitted, then it is. So really, light doesn’t travel straight through anything; the material just collects the light and then sends the same type of light out the other side. Think about that next time you look out the window.


by Dr Chris Russell


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