Tl;dr most objects that are transparent are electrical insulators. Being an insulator means having large band gaps which prevent absorption of incident light waves. Structure is also very important in deciding the transparency of an object, so much so that non-pigmented insulators which are opaque reflect light not because they are absorbents of light like metals, but because they have structures which force them to refract light a lot, causing them to appear opaque. If you take a sufficiently thin slice of most insulators, it would be transparent. It is their thickness which gives them opacity and colour.
We recommend you read our article on glass before reading this article.
Take a sheet of glass, or take a water glass instead, or maybe a glass cube. No matter how much you change the size of these various glass objects they will remain clear and colourless.
Now take a glass full of water and on the other side of your brain, imagine yourself scuba diving in a big ocean. If you haven’t noticed yet, the water in the glass is still transparent as the glass itself. But that big ocean you’re diving in is blue all around. How can water be both transparent and blue respectively?
One more thing: try looking through that glass of water and also ahead in the ocean. Notice how far you can see in both cases? Is there a difference in the visibility?
Look at the sky and then around yourself. Does the air around look blue as the sky? No. It is absolutely clear! But how?
Just wonder how a chair in your room can still look the same colour and the same way no matter how you see it or where you take it but water can be blue or clear at different times.
Most of Physics revolves around electrons doing stuff. They are always in the lime-light. The answers to the above questions and the concept of transparency are pretty much electrons behaving differently with different wavelengths of light. Building on our previous article on Colour, in this one, we explain transparency and its causes.
Types of Materials
Most objects we see around have shadows because they are opaque. They absorb the light that falls on them and hence block it. Whereas, things like glass, water, or air, let the light pass so that you can see through them. Some materials block the light completely while some partially (translucency), while some no light at all.
Transmission of Light
Transmission is what happens in a transparent object. In this light passes without any absorption or scattering through an object. But it does not mean that the material could not change the direction of light. Some materials are optically denser (to light) than others. Just like you can run more easily through air than in water, same goes with light. When this happens, light wave decelerates and so its direction changes. We call it Refraction of light. When a light ray transmits, it does not interact with the atoms or electrons of the medium, it has to pass from in between the spaces. When it enters a denser medium, clearly it will have to pave a new path by bending as the atoms are now much closer than before or the opposite. Refraction happens at every interface of different media.
Read here for more insight.
Refraction is the reason you can still see the shadow of a transparent glass of water.
Why do some objects transmit light while others don’t?
How an object responds to an incident wave
Let’s first see what happens when light enters an object. We are talking about simple elemental objects with no complex molecules and such. This light has to interact with the electrons of that object. Electrons are the most probable places the energy of the light can be used. If we look at an atom, electrons are the lightest subatomic particles which makes them capable of conducting various things like heat, electricity, etc. So why not light?
When light enters an object, the interactions happen like everything is a wave. When these electrons sense an external wave coming (the light wave), they experience an external oscillating field. Now we know light is an electromagnetic wave and electrons are charged. The electrons in the object get a reason to break Newton’s First Law and oscillate with the external field. So the electric field of the incident EM waves is capable of making the electrons align themselves with it (take the case of a microwave, for instance). The extent to which an object’s electrons respond to these waves determines the optical properties of that object. The more freely and alike the electrons are able to oscillate with the external wave, the more opaque the object becomes to that wave. This is because they keep using that wave’s energy to continue their own oscillations, thereby preventing it from passing through.
Fresnel Equations are a way to mathematically determine how an object will respond to light. When light strikes the interface between a medium with refractive index n1 and a second medium with refractive index n2, both reflection and refraction of the light may occur. The Fresnel equations describe the ratios of the reflected and transmitted waves’ electric fields to the incident wave’s electric field (the waves’ magnetic fields can also be related using similar coefficients). This ratio is called reflectance / transmittance, which is a complex number whose real part describes the usual “optical density” causing refraction (the more optically dense, the more opaque), and the imaginary part gives the measure of absorption of light by an object (remember, absorption causes opacity). Both of these parts need to be low so that the object is able to transmit light efficiently.
Optical Nature and Band Gaps
Bands are collections of a close range of energy levels and are formed when a lot of atoms come closer and their energies overlap. So band gaps exist for the system as a whole and not one atom. There are two main bands in every material, a Valence Band and a Conduction Band. Valence band is a pool of unexcited electrons which are not very free to move around.
As soon as we provide enough energy to these electrons they jump to the conduction band where more motion is possible. This all relates to transparency in the sense that not every object has electrons which can be readily influenced by the incoming light waves, like we discussed in the previous section. These electrons have to be made to cover their respective band gaps so as to start their own oscillations with the EM waves. Not everyone is “Metals”. This energy is provided by the light waves themselves. But electrons are very choosy in accepting the energies, which is why every object has a different colour, every object conducts heat differently, etc. Once the light’s energy is just right for an electron to cover that band gap, it absorbs the light and takes the leap. Most of the materials find visible light’s various wavelengths enough for covering their band gaps, which is the reason most of them are coloured. So, what’s different in glass?
Glass requires very high energy to make through and visible light is simply not capable of that. Although glass absorbs and oscillates due to frequencies higher than the visible range, since what we can see is not absorbed, it cannot be scattered to reach our eyes. So light just passes through the glass and it appears transparent.
Reasons for the very high band gap of Glass (or transparent objects)
Non-conducting materials which have trouble moving their electrons even when a large potential difference is provided will of course be unable to do so with the still lesser energy of visible light. Materials like glass, water, or polymers like polyethene are such insulators and so are mostly found transparent. But remember, being an insulator and transparent do not go hand in hand. A transparent object is always a bad conductor of electricity but the opposite may not be always true. Sometimes insulators may contain additional substances which have their own absorption spectra like how wood and rubber are coloured despite being insulators (See here). The Band Theory also indirectly explains why we do not really see transparent metals.
What affects Transmission (and also Transparency)
Our point of explaining the resonance condition of electrons in objects, their band gaps, and whether their conductivity affects transparency, was to bring light to the Physics behind Transparency.
This Physics fails to be visible when certain factors interfere with it.
1) Bulk and Regularity
Refraction plays a major role in deciding the fate of an otherwise transparent material. Every object is made up of a huge number of atoms or molecules. The way they are arranged in that object’s lattice affects its appearance. When atoms arrange themselves in random or conflicting directions, then light also starts bending accordingly in random directions. The rays that were passing as bent but straight (like in a glass slab), now spread in all directions, some of which may come back to our eyes and since they are from a transparent object and contain all colours, the object may appear white.
This is the reason why Ice is clear but Snow is white. Read this interesting article.
Or how, Indium Tin Oxide (ITO) manages to be both a conductor and a transparent material for your smartphone screen.
Indium Tin Oxide (ITO)
We just discussed how a transparent object is always an insulator. It’s time we redefine the statement a bit. Transparency and insulation can co-exist only when the material is intrinsically non-conducting (purely, an insulator).
ITO contains 80% indium oxide (in insulator) which has a band gap much greater than visible light could cover so indium oxide just lets it pass through, giving ITO transparency. But remember, doping a material with other materials having different properties can have a different overall effect. So here comes 20% of tin oxide which turns the combination of indium tin oxide into a semiconductor.
ITO is used in very thin layers as being a conductor, large quantities cause more reflection due to more free electrons. Also, in bulk quantities structure and refractive index of material lead to more scattering. We will discuss scattering effects in our last factor.
2) Structure: Interesting Plastics
Structure alone can determine the chemical properties of an object without any chemical changes. Plastics are man-made polymers and some of them are transparent too. This can provide us a direct encounter to understand how they get their optical properties. While most plastics (polymers) have the same energy reasons for being transparent or opaque, a more important factor in their case is the arrangement of molecules.
Amorphous and Crystalline Solids
Now that you know the difference, let’s talk what makes most amorphous solids transparent. Think of them as a liquid with a definite shape (not actually a liquid though). So when a light wave enters its structure, it would not have rigid obstacles having it bend its path very often. This mostly allows a clear passage for the light to transmit. See here why structure here is more important than the object being naturally transparent.
3) Scattering Intensity
We can see through most liquids around us. It’s because they have a highly regular structure, so refraction is quite smooth. But they mostly have a characteristic colour too, like water is blue (didn’t see that coming), or orange juice is, well, orange, but you can still see through them. These liquids allow the partial transmission of light and are called to be translucent. Translucency works just like objects get their colour, only that it shows both behaviours at times. Translucent objects absorb certain light wavelengths like other coloured objects do. But instead of scattering the rest of the light, they let it pass through.
Now, think why sky appears blue but the air around you is extremely clear. The air’s composition is the same throughout. Its absorption spectrum has a tendency to absorb bluer wavelengths and then re-emit to scatter them in all directions. (Please note: Absorbing a wavelength and completely eliminating it from the incident light is different from absorbing a wave to re-emit it, that’s what we call scattering.) When you look at the sky, air is the only thing between your eyes and the incoming light. So all you see is the blue colour scattered by the air. (Reference)
But when you look around, air is not the only thing absorbing the light. There are far more dense objects like a wall or a chair around that absorb this light. The intensity of the light scattered by these objects overpowers that by the air. Also, the more scattered the light from a single source is, the less intense it gets. So the distance between the sky and our surroundings also matters.
Read more about how to experience the blue sky effect in your surroundings.
Another example to understand the effect of scattering is how a small glass of water is much transparent and colourless than a big swimming pool having slightly blue water. (Read: Colour of Water)
So even if you have a naturally opaque object like wood, just make it thin enough to reduce the intensity of its absorption and scattering such that it becomes unnoticeable. All of us with siblings do something like this all the time!
So whenever you come across any object, ponder if it is originally the way it appears or are there certain things making it look that way. ¯(ツ)/¯