99.9% OF ATOM CONTAIN NOTHING

 Q: If the atom contains 99.9% of nothing, then why aren’t we just a tiny bit transparent then?

There are two ways of answering this, both of which are equally valid.

The first is that it’s a myth that atoms are mostly empty space. The myth arises from thinking of electrons, neutrons, and protons as little balls, with the electrons orbiting like planets around the sun. That’s still the way many children are introduced to atomic theory, but it’s utterly wrong.

In fact, these “subatomic particles” are not literally particles (the way we understand them in everyday life) nor are they waves (the way we understand them in everyday life). They are what they are—quanta—and have both wave-like and particle like properties, but are not either of those.

The result is, they don’t have fixed sizes and positions, but “spread out” through space in a probabalistic way.

That is, the space isn’t empty, but is occupied by the “particles” as tightly as possible, given their wobbledy natures.

The second consideration is that light is also made of quanta and these have a wavelength a thousand times bigger than atoms. So light cannot “enter” any atom, it can only interact with the outermost shell of electrons. Whether the inside of the atom is “empty” or “full” is irrelevant.

Whether matter is opaque or not and what color it has is determined by whether and how photons of light interact with the outermost layers of electrons in the atoms of that material. If photons arrive at a sufficiently wrong frequency (energy) to be absorbed by the outer electrons, they will mostly pass right on through the atom, and whether that space is “full” or not makes no difference.

This is why x-rays can pass through people and landscape images through windows—it’s just a matter of the photons not being absorbed.

It’s also why radio signals can’t penetrate deep underground. Radio is just light of a longer wavelength, and can penetrate most common materials. But like all quanta, its frequency and interaction is probabilistic. Even a photon of “the wrong frequency” has a small chance of being absorbed, though as the barrier grows thicker, that chance grows larger. Eventually, no photons can make it through any farther.

On the other hand, while light transmission occurs when the energy (wavelength) of the light is much larger or smaller than that the electrons can absorb, reflection or scattering occurs when the energy is close to the absorption band but absorption doesn’t occur. The classic reason for this is in a polished metal, where surface electrons are loosely bound enough to absorb photons of essentially any energy, but then bump into a neighbour and release the just-absorbed photon. This is why the bulk forms of metals are generally grey, and why polished metals (where there are no surface irregularities to trap escaping photons) make good mirrors.

But again, this is all probabalistic, so for example, even the best mirror absorbs some photons of all frequencies. A polished piece of wood may be shiny, but because it doesn’t have that outer sea of conductive electrons, it’s not exactly a mirror. And a leaf, having both a rough surface attuned to absorbing the magenta end of the visible spectrum may not be shiny at all, but still reflects enough green photons to have color. And of course, all three examples gain nuance through some degree of absorption, transmission, scattering, and reflection—nothing is every really perfectly reflective, transparent, or opaque.