Why are metals opaque and ceramics transparent?

If you make a metal thin enough it will become transparent; if you make a ceramic thick enough it will become opaque. It's a function of the wavelength of light and the thickness and which wavelengths get absorbed or are transmitted.

Whether a material is opaque or transparent is determined by whether or not a (visible light) photon will be absorbed, reflected or transmitted by the material. The explination is quite lengthy but consider this example: sillicon is opaque to visible light but it is transparent to infra red light. Glass (sillicon dioxide) is transparent to visible light, but opaque to unltra violet (ie: you won't get a sunburn if you stand infront of a window).

Dwayne

Overlysimplified:

The conduction or outermost electrons in the metal are "free" or unbound and you can sense that by the high electrical conductivity of the metals.

These "free" electrons will interact with the electromagnetic field of light.... by absorbing it.... and the metal is opaque.

The conduction or outermost electrons in the ceramic are bonded to other atoms. They are not "free" and won't ordinarily interact with visible wavelengths of light (electromagnetic wave).

But another reason that ceramics are opaque is internal light scattering from porosity and second phases (with differing indices of refraction) and grain boundaries.

All of this scattering will usually make a nice white opaque ceramic. Even if the small crystals of the ceramic are otherwise transparent.

The "electromagnetic field of light"??? Please explain.

Maxwell's equations of electromagnetics are a classical basis of light propagation, and diffraction ...... and light is an electromagnetic field (wave), in the classical sense.