Why You Can See Through Sand - band gaps, glass, metal and silicon

Every photon on that ruler carries one fixed-size kick of energy. Whether a material is clear or opaque is whether those kicks are big enough to be used.

Before you start. You need two ideas. From Your Remote Throws Invisible Light - the EM spectrum: each colour of light comes in packets called photons, and a photon’s energy, in electron-volts (eV), is 1,240 divided by its wavelength in nanometres, so red light is about 1.8 eV and violet about 3.3 eV. From Three Lights Pretending to Be Every Colour - screens, eyes and cameras: a camera sensor is silicon, which counts photons down to about 1.1 eV. That second fact is half of this lesson already.

The idea in 30 seconds

A material absorbs light only when a photon’s single kick of energy can lift an electron across the material’s band gap in one jump.
Glass’s gap is about 9 eV, far above visible light’s 1.8 to 3.3 eV, so pure glass is transparent; impurities, not glass, made early fibres lossy.
Kao and Hockham’s 1966 paper argued pure glass could lose under 20 dB per kilometre, and that became fibre optics.
Sand is the same clear silica; a pile looks white because its millions of surfaces scatter light, not because it absorbs it.
Metals have no gap, so they absorb and re-emit every visible photon at the surface: opaque and shiny.
Silicon’s gap is about 1.1 eV: it absorbs all visible light, which makes it a good camera sensor, and it is clear to infrared.

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