Is there any microscopic reason why some objects are transparent and some are opaque?

Question

I appreciate your replies.

Answer 1

Yes, it does have to do with the microscopic structure of the object. Light interacts with charged particles like electrons. Matter has electrons in abundance. Accelerated electrons produce light. Above all, energy is conserved.

So: light incident on an electron causes it to wiggle and radiate. The energy of this scattered light has to come from the incident beam. Since there are enormous numbers of electrons in any macroscopic piece of matter, why isn't everything opaque?

One way to make something transparent is to make it extremely thin. This obviously reduces the number of electrons available for scattering. If you imagine the incident light as a stream of photons, then a greater ratio of photons to electrons improves the chances of some getting through unscathed. Likewise, you can see bright enough lights through almost anything--at least before it melts.

But what about thick pieces of transparent stuff? The answer has to do with the bonding of the electrons. You see, the electrons' ability to wiggle in response to any incident frequency depends on how they are bound into the material. They may be localized--like being on a stiff springy leash--or they may be free to move throughout the material.

Free electrons can wiggle freely in response to any stimulus, and so materials with many free electrons tend to be opaque. This is why metals, which are good electrical conductors (eg the electrons are free to move) are opaque.

Bound electrons can only wiggle over certain ranges of frequencies, determined by the specifics of how they are bound. Depending on the frequency (color) of the incident light, they will scatter efficiently or almost not at all. This is the origin of the (reflective) color of many objects.

Now light comes in frequencies (colors) that we cannot visually perceive. Ultraviolet, radio, gamma, etc. The reflective color of transparent objects is out of the visible range. Ordinary glass windows are transparent to visible light--they scatter it only weakly--but they are opaque to UV. This is why you can't get a suntan in a greenhouse. Metals (it turns out that the electrons are not completely free) are transparent in the UV.

Answer 2

to simplify the issue for you, lets take an example which is sometimes is transparent, and in other cases opaque,

ICE

Sometimes you see the ice sometimes transparent and sometimes opaque,

the reason is the structure of the Molecules, if it is in order manner then the object will be opaque, if it is random then it is transparent,

this applied on everything

Answer 3

yes, but different objects are opaque or transparent at different frequencies (colors) of light.

if a molecule's electromagnetic field can resonate or vibrate at a given frequency, and cause other nearby molecules to do the same, the object is transparent.

otherwise, the object is opaque, for that given frequency.