What is the cause of "transparancy"?

Question

What properties/features of an element(at atomic level) causes some material to behave as 'transparent' and some to behave as 'opaque'? assuming we are comparing the material samples to be of standard thickness say 1 inch thick each.

Answer 1

The interaction of visible light with matter can be thought of as individual photons with a certain energy interacting with the electrons in the material. If we take green light (wavelength = 555nm) as an example, each photon carries 2.2eV of energy. If the material can absorb this amount of energy in a single chunk, then the material will be opaque to this wavelength.

Surprisingly, however, it is often the case that the electrons can not absorb 2.2eV all at once because there are no unoccupied electron states located 2.2eV above the ground state. For example, think of a gas of hydrogen. The lone electron of a hydrogen atom sits in the 1S orbital. The next orbital up in energy is the 2S, It takes about 10.2eV to excite an electron from the 1S state into the 2S state. If a photon carries less than 10.2eV, it will pass right on by the hydrogen atoms. i.e. it will be transparent. If it carries close to 10.2eV, a hydrogen atom might have a good chance of absorbing this photon, and enjoy a good old time in the 2S state for a little while. i.e. it will be opaque.

Things get more complicated when you consider dense matter where the energy levels of the electrons get all munged together due to interactions with close-by neighbors, but the same logic applies.

Also, there are more subtle effects than the simple absorbtion of a single photon by a single electron. There are cooperative effects in which multiple photons and/or "collective states" of many electrons take part. But again, the same logic applies.

Answer 2

If an atom is hit by a photon (light particle), with a specific energy, one electron absorb the lights energy and will enter a higher energy state. The electron will quickly fall back to it's original state and emit a photon, but that photon will be emitted in a random direction, so that is why we can't see through that material.

In transparent material the electrons don't have any higher energy level that fits the energy of the photon, so they can't absorb the photon and the light passes right through.
There may also be other reasons why materials are opaque. They may for instance be made up of many crystals, that in them selves are transparent, but the light will be reflected at the surface between the crystals.

Answer 3

Hi. Some frequencies of light can penetrate some materials for some distance. (Nothing is truly transparent.) You may not see through an object in visible light, but heat (infrared) and x-rays might pass right through. Glass lets visible light frequencies pass but blocks ultraviolet. I think this is do to the interaction of photons with the material.

Answer 4

When light interacts with matter, it can be reflected, absorbed,
scattered, or transmitted. An object is generally described as
"transparent" if a significant fraction of the incident light is
transmitted through the object. An object is considered "opaque" if very little light is transmitted through it. And object is considered
"translucent" if some light passes through but not in a way that a
coherent image can be seen through it.