If we had never invented transparent materials like glass, we might not believe that such magical invisible materials - that are both solid and see-through - could really exist.
Yet glass is a technological reality, made from rather mundane earthly kinds of ingredients like sand and limestone. These are, of course, entirely opaque, made up of atomic elements such as silicon, sodium, calcium and carbon. And these atoms are all made of even smaller particles like protons and electrons, each of which is so small as to be invisible in the first place.
Of course, when we look at an atom close up, we realise that there is very little there to see anyway: it is mostly empty space. The size of the atomic nucleus relative to the diameter of the whole atom is similar to the size of a tennis ball compared to, say, the diameter of a medium-sized town. And a town consisting of a tennis ball surrounded by empty space would be a town you could see right through.
Indeed, this emptiness pervades all matter around us that is built from atoms. Almost all of the volume of our towns, tennis balls, cats and cream cakes is just empty space: completely void, save for a few microscopic specks of matter dusted here and there. In a sense, it is a wonder that we, and everything around us, are not all completely see-through.
(If everything actually were see-through, of course, we wouldn't be able to see anything at all. We couldn't even see the insides of our own eyelids, from the backs of our retinas).
The difference between substances that are transparent and those that are opaque is to do with their structure at the molecular level. In other words, the different appearance of glass and sand or coal and diamond is due to their different molecular structures, even if these are made up of the same kinds of atoms, which in turn are made up of a kit of the same sub-atomic particles like neutrons or quarks. So transparency and opacity are effectively properties of the built structures, rather than properties of the individual parts in the building kit.
The historic discovery that the stars and planets are made out of the same kinds of elements found on Earth was a significant breakthrough in our understanding of the universe. On the one hand, this made the universe more familiar, and its component parts more, well, universal. On the other hand, it also opened up the possibility of outlandish life-forms inhabiting other planets, which were now understood to be other 'worlds' rather than just heavenly nightlights. As a result, we tend to be more imaginative with our alien biology than our alien chemistry.
If we were visited by an unidentified flying object made out of some strange invisible-looking material, we would most easily believe that it was the technological product of an alien life-form, rather than imagine that objects from outer space were made from some special extra-terrestrial substance - invisibilium, let's say - that was alien to chemistry.
We could even imagine the aliens themselves could be invisible - or at least, evolved to be more or less transparent, like extra-terrestrial jellyfish or fibre-optic stick insects.
In fact, successfully adapted invisible aliens could even travel in their invisible spaceships to settle new planets. We could imagine just a few precautions they would need to keep as invisible as possible, if they came to live on Earth.
For a start, an otherwise invisible alien would have to watch out that it would not cast a refractive shadow, if its body were to bend or scatter the light in an unearthly manner. Then, it should avoid smoky bars, in case its breathing created telltale smoke signals in the atmosphere. And, it should avoid attracting too many flies, bouncing off its invisible body.
An invisible immigrant should take particular care when eating out. Imagine that our alien has found itself a nice piece of fruit, packed with tasty seasonal insects, or some leftover human meal, say a pizza with extra toppings of maggots and fungus. The danger is that once ingested, the terrestrial food will remain visible as it passes through the transparent alimentary tract, so that the alien becomes horribly visible from the inside out: a walking squelching sack of bile and compost-fodder. To remain decently invisible, the alien would be well advised to bring its own invisible food chain, from home.
And if successfully transparent - magically solid but see-through just like glass - then invisible aliens need not be confined to science fiction.
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