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Originally Posted by MalaMan
Thank your for the answers so far.
I'm really intrigued by the realization that "opacity" and "transparency" are properties of materials that are related not only to visible light, but also to all other wavelengths of electromagnetic radiation.
Take a wall made of bricks, for example. It may be "opaque" to electromagnetic radiation in the wavelengths of red light, and blue light, but it can be "transparent" to electromagnetic radiation in the wavelength of AM radio (it's perfectly possible for an AM radio receiver inside a closed room with brick walls to detect the electromagnetic "signal" from an AM radio station). In another example, muscle tissue of animals may be "opaque" to electromagnetic radiation in the wavelengths of red, green and blue light, but it's "transparent" to hard X-rays. In contrast, bones are mostly "opaque" to hard X-rays too....
It seems to me that the factors that determine the properties of "opacity" and "transparency" of each material to each wavelength of electromagnetic radiation are very complex factors.
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The spectrum of waves and wave frequency IS complex, too much so for easy glib answers. At best, I can only offer some starters to get you headed in the right direction:
In a wave, physical material is not propagated in the direction of wave travel. Crudely stated, the material is compressed and decompressed slightly in a unidirectional manner. It does so because a wave has inherent energy (there was a movement that initiated it), and it takes far less energy to pass that movement along than to stop it.
Take a fifty foot rope, tie one end to a tree, pull the rope taught and move it up and down quickly at the other end. You will see the waves propagate down the length of the rope, while the fibers of the rope are still at the same position on the rope. You may also see the reflection of the wave come back from the end tied to the tree if the system is taut enough that there isn't damping of the wave from slack.
What happens when you tie the end of the thin rope to a thicker rope? Play with it to see.
Next, make a cigar box violin and learn how materials have resonance, and certain frequencies of waves are accentuated or attenuated because of the resonance and material properties. Again, the wave propagates without any massive distorting of the shape of your violin.
Move on to building and playing with a simple wave table (big pan with a layer of water in it). See how point source waves behave, then use it to learn about refraction and reflection. Do a search on "wave table experiments" for ideas.
Move on to electricity. You are about to learn the difference between Edison and Tesla. With electricity there are two components involved, the wave (which as stated before has energy) and electrons. Electrical power is (again crudely stated) the energy that can be tapped when electrons move.
With direct current, there is no wave, and electrons are passed from molecule to molecule like a bucket brigade. In alternating current, a wave is induced into the conductor, which then acts much like your rope in the first experiment. The energy in the wave moves each electron in the path only a tiny amount (which is VERY energy efficient) The electrons go one way, then the other, repeatedly.
Direct current has losses and resistance all along the way. Alternating current has very low losses, and the energy put into the wave at one end can be retrieved at the other by anything that damps the wave.
Of special note is that with electricity you are dealing with two distinct things an (arguably physical) electron AND a wave.
Next, remembering the basics of electricity, move on to light. Light is both a wave and a photon, but a photon is more strange than an electron, which usually circles around a nucleus in various energy states.
In a way, a photon is and isn't at the same time. When we get light from a star billions of miles away, that light travels as the efficient wave energy. It doesn't travel as a particle. Particles cannot travel at the speed of light, but only approach it with the use of huge amounts of energy.
When the wave that is light encounters a material that damps it, (AGAIN, very crudely) the photon (physical representation of the light) manifests. So, much like electricity, light is a combination of the properties of different elements.
Light can pass through glass or many crystals, as the glass is mostly able to keep the wave intact because of the structure of the material.
When light of a certain frequency cannot maintain the wave, and the wave is damped, the energy of the wave gets released, just as with alternating current. The easy example is when you stand in bright sunlight and get warm, because all of the infrared wavelength waves are hitting your body and going "Oh s**t! I can't get through!" All those little swear words from the infrared waves add up and turn into heat (which is another definition of localized energy).
There is regular window glass, but there also is a special heat absorbing glass. The absorption is achieved either through adding colorants that damp certain wavelengths of light, or metal oxides that are commonly thought of as transparent, but are slightly darkening in the visual spectrum.
This all leads to the core answer to your question about transparency. If a wave passes through unimpeded, a substance is transparent. If a wave isn't blocked but there is a lot of refraction in different directions, a substance is translucent. If a wave is completely blocked, a photon appears, energy is released, and depending on the vibration speed of the child wavelengths, various colors manifest.
An X-ray usually passes through water but is stopped by bone, where the wave releases its energy or is reflected. The shadow of the bone projected onto a plate forms an image. The released energy of the X-ray, when focused on particular cells, can disrupt and kill or mutate them.
The above is all VERY crude and intended for basic understanding. Dive down into any of it and you will find it much more complex and even confusing.