# What makes glass transparent?

A physicist is allowed about five minutes to explain why some things are transparent to visible light:

The basic idea is that when a photon is absorbed by a substance, an electron in the substance jumps from one energy level to another, corresponding to the amount of energy of the photon.

But it turns out that electrons can only exist in certain energy levels. (That discovery was what launched the entire branch of physics called quantum mechanics.)

And in the meantime, the energy of a photon depends on the wavelength or frequency of the light in question. If you imagine watching waves go by at a given speed, it's obvious that the shorter the distance between the wave crests, the more of them go by in a second. In fact, the wavelength times the number of waves per second (the frequency) equals the speed at which the waves go past.

It seems reasonable to suppose that the more waves per second, the more energy they carry, and that's true. Another way of saying it is that the shorter the wavelength, the higher the frequency, and the higher the energy level.

So since an electron can only exist in certain energy levels, it absorbs a photon only if that photon has the right amount of energy to take it up to the next energy level it can occupy. Otherwise the photon just keeps sailing on past, ignored. In transparent glass the possible electron energy levels are so far apart that photons can't be absorbed. In metals, on the other hand, electrons can occupy a lot of energy states, entire "conduction bands," so they absorb pretty much any photon that tries to get through. Other substances are in between. A brick for example, will typically absorb photons of visible light (unless it's a glass brick) but allow radio waves to pass right through. Radio waves are light, just with a very long wavelength and very low-energy photons.

Incidentally, this video is from the University of Nottingham, which I assume is where Robin Hood did his undergraduate work.

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#### What makes glass transparent? — 4 Comments

1. So what would a neutron star or anything made of neutronium look like, assuming there are no free electrons?

• This is an interesting question to which I've seen inconsistent answers. An interesting article on this and related subjects appeared in the American Journal of Physics. You can download the paper in PDF form from this link. I seem to recall that there's also some mention of the subject in physicist Robert L. Forward's novel Dragon's Egg, and possibly also in the sequel, Starquake, both of which are considered "hard" science fiction, in the sense of based on accurate science.

My impression is that neutron stars tend to be extremely hot and hence emit black-body radiation in X-ray wavelengths (on top of X-rays caused by an inflow of accreted material from a companion star), but also in the visible part of the spectrum. I've read assertions to the effect that at least some neutron stars give off little or no visible light, but others clearly do, at least one having been imaged by Hubble. Especially in the case of neutron stars with intense magnetic fields (magnetars) I'm sure some emissions are due to acceleration of charged particles such as electrons, but my admittedly inexpert guess is that there would still be simple thermal emissions as with any other object. There's more discussion here and here (the latter of which asserts, "Even at [as low as] 1 million kelvin[s], most of the light generated by a neutron star is in X-rays. In visible light, neutron stars probably radiate approximately the same energy in all parts of visible spectrum, and therefore appear white.".

• Gary, thanks for the detailed reply and links. I'll check 'em out. I was thinking less of high-energy or blackbody radiation and more like, what would you see if you shone a light on a piece of neutronium, or anything that didn't have very many, or any, electrons in it.

• That's a very good question to which I don't know the answer. Last night I mentioned the original neutron star question to a physicist I know and he didn't have a quick answer either. I did run across another paper on neutron stars that mentions (bottom of page 19):

A neutron star can be considered as having ﬁve major regions, the inner and outercores, the crust, the envelope and the atmosphere. The atmosphere and envelope contain a negligible amount of mass, but the atmosphere plays an important role in shaping the emergent photon spectrum, and the envelope crucially inﬂuences the transport and release of thermal energy from the star’s surface.

So the visual properties of a neutron star don't necessarily tell us much about neutronium as such. I have an impression that photons (even high-energy gamma rays) don't generally interact with neutrons, but would that make a dense mass of neutrons transparent like glass? That seems wildly counterintuitive, but then again our ordinary intuition is pretty useless when it comes to physics on this level anyway. As I said, it's a good question!